Explanation
Selecting the 'Use for triggered send' checkbox is the correct answer. This option allows you to create a data extension that can be used as a target audience for a triggered send. A triggered send is an email that is sent in response to a subscriber's action or behavior, such as a purchase confirmation or a welcome email. When you select this option, you also need to include the subscriber key and email address fields in your data extension.
Creating from template and choosing triggered send data extension template is not a valid option, as there is no such template available in Marketing Cloud. Including subscriber key and email address field as primary key is not required, as only the subscriber key field needs to be set as the primary key. You can learn more about this in the Data Extensions module on Trailhead.

Explanation
List-Unsubscribe header is a feature that allows subscribers to unsubscribe from an email by clicking a link in their email client. This link is added by Marketing Cloud to the email header and can trigger an unsubscribe reason related to RMM, even if RMM is not enabled in the account. [Source: Trailhead]

Explanation
Data Filter is a tool that allows marketers to create filtered data extensions based on criteria without writing SQL queries. Data Filter can also provide a quick count of records that match the criteria before creating the filtered data extension. [Source: Trailhead]