The correct answer is C. When events contain unstructured data.
The regular expression method works best with unstructured event data, such as log files or text messages, where the fields are not separated by a common delimiter, such as a comma or space1. You select a sample event and highlight one or more fields to extract from that event, and the field extractor generates a regular expression that matches similar events in your dataset and extracts the fields from them1. The regular expression method provides several tools for testing and refining the accuracy of the regular expression. It also allows you to manually edit the regular expression1.
The delimiters method is designed for structured event data: data from files with headers, where all of the fields in the events are separated by a common delimiter, such as a comma or space1. You select a sample event, identify the delimiter, and then rename the fields that the field extractor finds1. This method is simpler and faster than the regular expression method, but it may not work well with complex or irregular data formats1.
Reference:
1: Build field extractions with the field extractor - Splunk Documentation

When you define a calculated field, you can specify the name of the field that the eval expression will create or modify. If the name of the calculated field matches the name of an existing extracted field, the calculated field will override the extracted field and replace its value with the result of the eval expression. This means that the original value of the extracted field will not be available for searching or analysis. To avoid this, you should use a unique name for your calculated field or use a different name for your extracted field2
1: Splunk Core Certified Power User Track, page 9. 2: Splunk Documentation, Configure calculated fields with props.conf.

The Splunk Common Information Model (CIM) add-on is a collection of pre-built data models and knowledge objects that help you normalize your data from different sources and make it easier to analyze and report on it3. The CIM add-on includes pre-configured data models that cover various domains such as Alerts, Email, Database, Network Traffic, Web and more3. Therefore, option B is correct. The CIM add-on also includes fields and event category tags that define the common attributes and labels for the data models3. Therefore, option C is correct. The CIM add-on does not include custom visualizations or automatic data model acceleration. Therefore, options A and D are incorrect.

The searches A and C show a valid use of a macro. A macro is a reusable piece of SPL code that can be called by using single quotes (''). A macro can take arguments, which are passed inside parentheses after the macro name. For example, 'makeMyField(oldField)' calls a macro named makeMyField with an argument oldField. The searches B and D are not valid because they use double quotes ("") instead of single quotes ('').