The object that can be used to set a field to read-only in all applications of the RESTful Application Programming model (RAP) is the behaviour definition. The behaviour definition is a CDS artefact that defines the business logic and the UI behaviour of a business object. A business object is a CDS entity that represents a business entity or concept, such as a customer, an order, or a product. The behaviour definition can specify the properties of the fields of a business object, such as whether they are mandatory, read-only, or transient. These properties are valid for all applications that use the business object, such as transactional, analytical, or draft-enabled apps12. For example:
* The following code snippet defines a behaviour definition for a business object ZI_PB_APPLICATION. It sets the field APPLICATION to read-only for all applications that use this business object:
define behavior for ZI_PB_APPLICATION { field ( read only ) APPLICATION; ... } You cannot do any of the following:
* A. Service definition: A service definition is a CDS artefact that defines the interface and the binding of a service. A service is a CDS entity that exposes the data and the functionality of one or more business objects as OData, InA, or SQL services. A service definition can specify the properties of the fields of a service, such as whether they are filterable, sortable, or aggregatable. However, these properties are only valid for the specific service that uses the business object, not for all applications that use the business object12.
* C. Projection view: A projection view is a CDS artefact that defines a view on one or more data sources, such as tables, views, or associations. A projection view can select, rename, or aggregate the fields of the data sources, but it cannot change the properties of the fields, such as whether they are read- only or not. The properties of the fields are inherited from the data sources or the behaviour definitions of the business objects12.
* D. Metadata extension: A metadata extension is a CDS artefact that defines additional annotations for a CDS entity, such as a business object, a service, or a projection view. A metadata extension can specify the properties of the fields of a CDS entity for UI or analytical purposes, such as whether they are visible, editable, or hidden. However, these properties are only valid for the specific UI or analytical application that uses the metadata extension, not for all applications that use the CDS entity12.
References: 1: ABAP CDS - Data Definitions - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - Behavior Definitions - ABAP Keyword Documentation - SAP Online Help

The statement that can be used to expose the field "connid" of the data source "demo_ods_assoc_spfli" in the element list is A. demo_ods_assoc_spfli.connid,. This statement uses the dot notation to access the field
"connid" of the data source "demo_ods_assoc_spfli", which is an association defined on line #4. The association "demo_ods_assoc_spfli" links the data source "demo_ods" with the table "spfli" using the field
"carrid". The statement also ends with a comma to separate it from the next element in the list12.
You cannot do any of the following:
* B. demo_ods_assoc_spfli-connid/: This statement uses the wrong syntax to access the field "connid" of the data source "demo_ods_assoc_spfli". The dash notation is used to access the components of a structure or a table, not the fields of a data source. The statement also ends with a slash, which is not a valid separator for the element list12.
* C. spfli-connid,: This statement uses the wrong data source name to access the field "connid". The data source name should be "demo_ods_assoc_spfli", not "spfli". The statement also uses the wrong syntax to access the field "connid", as explained above12.
* D. _spfli.connid/: This statement uses the wrong data source name and the wrong separator to access the field "connid". The data source name should be "demo_ods_assoc_spfli", not "_spfli". The statement also ends with a slash, which is not a valid separator for the element list12.
References: 1: ABAP CDS - SELECT, select_list - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - SELECT, from - ABAP Keyword Documentation - SAP Online Help

The annotation @Environment.systemField: #LANGUAGE is used to assign the ABAP system field sy-langu to an input parameter of a CDS view or a CDS table function. This enables the implicit parameter passing in Open SQL, which means that the value of sy-langu will be automatically passed to the CDS view without explicitly specifying it in the WHERE clause. This also applies to the CDS views that use the annotated CDS view as a data source, which means that the value of sy-langu will be propagated to the nested CDS views (view on view)12. For example:
* The following code snippet defines a CDS view ZI_FLIGHT_TEXTS with an input parameter p_langu that is annotated with @Environment.systemField: #LANGUAGE:
define view ZI_FLIGHT_TEXTS with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from sflight left outer join scarr on sflight.carrid = scarr.carrid left outer join stext on scarr.carrid = stext.carrid { sflight.carrid, sflight.connid, sflight.fldate, scarr.carrname, stext.text as carrtext } where stext.langu = :p_langu
* The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in ABAP without specifying the value of p_langu in the WHERE clause. The value of sy-langu will be automatically passed to the CDS view:
SELECT carrid, connid, fldate, carrname, carrtext FROM zi_flight_texts INTO TABLE @DATA(lt_flights).
* The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in another CDS view ZI_FLIGHT_REPORT. The value of sy-langu will be automatically passed to the nested CDS view ZI_FLIGHT_TEXTS:
define view ZI_FLIGHT_REPORT with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from zi_flight_texts(p_langu) { carrid, connid, fldate, carrname, carrtext, count(*) as flight_count } group by carrid, connid, fldate, carrname, carrtext The annotation @Environment.systemField: #LANGUAGE does not prevent the possibility of overriding the default value with a value of your own. You can still specify a different value for the input parameter p_langu in the WHERE clause, either in ABAP or in another CDS view. This will override the value of sy-langu and pass the specified value to the CDS view12. For example:
* The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in ABAP with a specified value of p_langu in the WHERE clause. The value 'E' will be passed to the CDS view instead of the value of sy-langu:
SELECT carrid, connid, fldate, carrname, carrtext FROM zi_flight_texts WHERE p_langu = 'E' INTO TABLE @DATA(lt_flights).
* The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in another CDS view ZI_FLIGHT_REPORT with a specified value of p_langu in the WHERE clause. The value 'E' will be passed to the nested CDS view ZI_FLIGHT_TEXTS instead of the value of sy-langu:
define view ZI_FLIGHT_REPORT with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from zi_flight_texts(p_langu) { carrid, connid, fldate, carrname, carrtext, count(*) as flight_count } where p_langu = 'E' group by carrid, connid, fldate, carrname, carrtext References: 1: ABAP CDS - parameter_annot - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - session_variable - ABAP Keyword Documentation - SAP Online Help

The <source> clause in the CDS View Entity Data Definition can be used to specify the data source for the view entity. The <source> clause can accept different types of data sources, depending on the type of the view entity1.
* A database table from the ABAP Dictionary: This is a valid type of data source for a CDS View Entity Data Definition. A database table from the ABAP Dictionary is a table that is defined in the ABAP Dictionary using the keyword TABLE or TABLE OF. The name of the database table must be unique within its namespace and must not contain any special characters2.
* A CDS DDIC-based view: This is also a valid type of data source for a CDS View Entity Data Definition. A CDS DDIC-based view is a view that is defined in the Core Data Services using the keyword DEFINE VIEW ENTITY. The name of the CDS DDIC-based view must be unique within its namespace and must not contain any special characters3.
You cannot do any of the following:
* An external view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. An external view from the ABAP Dictionary is a view that is defined in an external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the external view must be unique within its namespace and must not contain any special characters4.
* A database view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. A database view from the ABAP Dictionary is a view that is defined in an external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the database view must be unique within its namespace and must not contain any special characters4.
References: 1: CDS DDL - DEFINE VIEW ENTITY - ABAP Keyword Documentation - SAP Online Help 2:
ABAP Dictionary Tables - SAP Online Help 3: CDS DDL - DEFINE VIEW ENTITY - ABAP Keyword Documentation - SAP Online Help 4: ABAP Dictionary Views - SAP Online Help