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
Send Preview is the tool that allows the marketer to review how different variations of an email will render in different email clients. It also enables the marketer to preview dynamic content and personalization strings for each subscriber in a data extension. Content Detective is a tool that scans an email for potential content issues, such as broken links, missing images, or spam triggers. Test Send is a tool that sends a test email to a specified recipient or a test data extension.

Explanation
Based on Subscriber Preview List or Data Extension is a Preview & Test Content Personalization option that allows marketers to preview how an email will look for different subscribers based on their personalization attributes and dynamic content rules. Marketers can select a list or data extension that contains the subscribers they want to preview, and then use the arrows or search function to navigate through the different previews.