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.