Scheduling an automation to refresh the filter activity each day is the best step to ensure the data extension includes newly added records, as it allows the marketer to automate the process of updating the filtered data extension based on the latest data in the source data extension. The automation can run at a specified time or frequency, or be triggered by an event. Checking the 'AUTOMATICALLY REFRESH UPON SENDING' checkbox or configuring the filter DE to auto-refresh daily in properties would not work for this scenario, as they only refresh the data extension when it is used as a target audience for an email send.

To ensure a data extension appears as a Recipient Test Data Extension in the Test Send menu, it must be created with the 'Is Testable' attribute. This designation allows the data extension to be used in test sends, ensuring that marketers can send test emails to individual subscribers in the data extension. This feature is crucial for validating how emails will appear to different segments of the audience.
References:
* Salesforce Marketing Cloud Documentation: Testable Data Extensions

Explanation
The Sender Profile is the setting that allows you to configure the From Name, From Email, and Reply Email for a send. This can help identify the line of business sending the message. The Sender Authentication Package is a feature that enables you to use your own domain in your email links, images, and reply addresses. The Brand Builder is a tool that helps you create a consistent look and feel for your emails, landing pages, and microsites. You can learn more about these settings in the Email Creation and Sending module on Trailhead.