Explanation/Reference: https://docs.splunk.com/Documentation/Splunk/8.1.1/Knowledge/SetupaGETworkflowaction

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

Explanation
The correct answer is C. A macro is a portion of a search that can be reused in multiple places.
A macro is a way to reuse a piece of SPL code in different searches. A macro can be any part of a search, such as an eval statement or a search term, and does not need to be a complete command. A macro can also take arguments, which are variables that can be replaced by different values when the macro is called. A macro can also contain another macro within it, which is called a nested macro1.
To create a macro, you need to define its name, definition, arguments, and description in the Settings > Advanced Search > Search Macros page in Splunk Web or in the macros.conf file. To use a macro in a search, you need to enclose the macro name in backtick characters (`) and provide values for the arguments if any1.
For example, if you have a macro named my_macro that takes one argument named object and has the following definition:
search sourcetype= object
You can use it in a search by writing:
my_macro(web)
This will expand the macro and run the following SPL code:
search sourcetype=web
The benefits of using macros are that they can simplify complex searches, reduce errors, improve readability, and promote consistency1.
The other options are not correct because they describe other types of knowledge objects in Splunk, not macros. These objects are:
A: An event type is a method of categorizing events based on a search. An event type assigns a label to events that match a specific search criteria. Event types can be used to filter and group events, create alerts, or generate reports2.
B: 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 convenience3.
D: An alert is a knowledge object that enables you to schedule searches for specific events and trigger actions when certain conditions are met. An alert can be used to monitor your data for anomalies, errors, or other patterns of interest and notify you or others when they occur4.
References:
About event types
About field aliases
About alerts
Define search macros in Settings
Use search macros in searches

The Splunk Common Information Model (CIM) Add-on includes a variety of data models designed to normalize data from different sources to allow for cross-source reporting and analysis. Among the data models included, Alerts (Option B) and Email (Option D) are part of the CIM. The Alerts data model is used for data related to alerts and incidents, while the Email data model is used for data pertaining to email messages and transactions. User permissions (Option A) and Databases (Option C) are not data models included in the CIM; rather, they pertain to aspects of data access control and specific types of data sources, respectively, which are outside the scope of the CIM's predefined data models.

Data model acceleration is a feature that speeds up searches on data models by creating and storing summaries of the data model datasets1. To enable data model acceleration, you must have administrative permissions or the accelerate_datamodel capability1. Therefore, option D is correct. Accelerated data models cannot be edited unless you disable the acceleration first1. Therefore, option B is correct. Private data models cannot be accelerated because they are not visible to other users1. Therefore, option C is correct. Root events can be accelerated as long as they are not based on a search string1. Therefore, option A is incorrect.