marking the field as the Primary Key is what a marketer should do to ensure the Email Address field values are unique in a data extension. A primary key is a field or a combination of fields that uniquely identifies each row in a data extension. If a field is marked as a primary key, it cannot have duplicate values or null values.
Marking the data extension as Sendable or using Email Address as Subscriber Key does not ensure uniqueness of field values.

The personalization string that should be appended as a parameter to identify the business unit in each of its links is %%memberid%%. This string returns the ID of the business unit that sent the email.
This way, NTO can track customer engagement by business unit. Option A is not a valid option, as
%%membername%% returns the name of the business unit, not the ID. Option B is not a valid option either, as %%memberstatus%% returns the status of the business unit, such as active or inactive, not the ID. You can learn more about this in the Personalization Strings module on Trailhead.

Content Detective is the correct answer. Content Detective is a tool that allows you to scan your email content for any words or phrases that may trigger spam filters or cause deliverability issues. Content Detective assigns a score to your email based on the number and severity of the flags it detects, and provides suggestions on how to improve your content. You can learn more about this in the Email Testing Tools module on Trailhead.

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.

Einstein Engagement Scoring is the correct answer. This tool allows you to use artificial intelligence to score your subscribers based on their engagement behavior, such as opens, clicks, conversions, etc. You can use Einstein Engagement Scoring to easily target your most loyal subscribers by filtering them based on their predicted engagement level or score. You can also use Einstein Engagement Scoring to segment your subscribers based on their predicted attrition risk or optimal send time. Google Analytics Audience is not a valid option, as this tool only allows you to measure and analyze web traffic and behavior, not email engagement. Path Optimizer is not a valid option either, as this tool only allows you to test different paths or content variations in a journey based on different engagement metrics, not target loyal subscribers. You can learn more about this in the [Einstein for Marketing Cloud] module on Trailhead.