The fields sidebar is a panel that shows the fields that are present in your search results2. The fields sidebar does not show all extracted fields, which are fields that are extracted from your raw data using various methods such as regular expressions, delimiters or key-value pairs2. The fields sidebar only shows selected fields and interesting fields2. Selected fields are fields that you choose to display in your search results by clicking on them in the fields sidebar or by using the fields command2. Interesting fields are fields that appear in at least 20 percent of events or have high variability among values2. Therefore, option C is correct, while options A and B are incorrect because they are types of fields that the fields sidebar does show.

The correct answer is D. field2 values are replaced with the value of the field1.
A field alias is a way to associate an additional (new) name with an existing field name. A field alias can be used to normalize fields from different sources that have different names but represent the same data. Field aliases can also be used to rename fields for clarity or convenience1.
When you create a field alias in Splunk Web, you can select the Overwrite Field Values option to change the behavior of the field alias. This option affects how the Splunk software handles situations where the original field has no value or does not exist, as well as situations where the alias field already exists as a field in your events, alongside the original field2.
If you select the Overwrite Field Values option, the following rules apply:
If the original field does not exist or has no value in an event, the alias field is removed from that event.
If the original field and the alias field both exist in an event, the value of the alias field is replaced with the value of the original field.
If you do not select the Overwrite Field Values option, the following rules apply:
If the original field does not exist or has no value in an event, the alias field is unchanged in that event.
If the original field and the alias field both exist in an event, both fields are retained with their respective values.
Therefore, if you create a field alias where field1-field2 and select the Overwrite Field Values option, and an event only contains values for field1, then the value of field2 will be replaced with the value of field1.
Reference:
About calculated fields
About field aliases
Create field aliases in Splunk Web

The timechart command in Splunk requires the _time field to be present in the dataset because it uses time as the primary axis for charting data. The _time field represents the time of events and is essential for commands that generate visualizations based on time, such as timechart. This command groups the events into time intervals and performs statistical functions on those time intervals. Without the _time field, the timechart command will not function properly.
Reference:
Splunk Docs - timechart command

Tags are a type of knowledge object that enable you to assign descriptive keywords to events. Tags can reference any of the following knowledge objects: extracted fields, field aliases, calculated fields, lookups, and event types. Tags cannot reference other tags or search macros. Tags are applied to events at search time based on the values of the fields that they reference2
1: Splunk Core Certified Power User Track, page 10. 2: Splunk Documentation, About tags and aliases.