The correct answer is C, "subjunctive". "The imperative" is usually used to order somebody to do something, so it is not that hypothetical, since it is going to happen sooner or later. "The indicative" is not hypothetical, it is about the facts. "Active" or "passive" doesn't really mean that it can or cannot be hypothetical, so the answer is C, subjunctive, because it shows us what would happen had something else happened, that is, we hypothesize about something.
