Did the brand campaign actually drive new sign-ups?
Measure impactHardProblem. Pinterest ran a brand marketing campaign (TV and online) in a few countries. Afterwards, sign-ups in those countries rose. Leadership asks: did the campaign actually cause it, and was it worth the spend? You can't A/B a brand campaign by user.
Before you reveal: say your answer out loud, as if you were in the real interview — get your reasoning across clearly first. There is no single correct answer: reading what the interviewer is really after and defending your own thinking is what makes an answer strong.