Sufficient assumption questions ask us what would make the argument correct, and necessary assumption questions ask us what the author must agree with if their argument is to hold. Effectively, necessary assumption questions are the argument version of a must be true- the right answer is something that must be true for the conclusion to hold.
Here the question asks what we must assume in order to justify the conclusion. That is, it is asking what must be true for the conclusion to be justified or correct: which is what a necessary assumption question asks.
