I think it would be A if not I am sorry
C. The mediator is the answer because even though the question is trying to gauge whether you know the meaning of the word mediator, it is also providing a grammatical solution to itself. None of the other words fit in that sentence correctly except for mediator, and also the word mediator (someone who is keeping peace between differing parties) fits the actions the sentence is proposing.
I would go with D but I’m not exactly sure
C or D,As as a pre-author myself those are both correct
