Seriously, I would have also picked C.

shreeman wrote:269.During the Renaissance, scholars were uncertain as to the location of Troy, and by the eighteenth century many historians doubted that Troy had ever existed.
(A) doubted that Troy had ever existed
(B) doubt that Troy has ever existed
(C) were in doubt as to the existence of Troy
(D) were doubtful concerning Troy's existence
(E) had doubts about Troy's ever existing

can anyone please explain this ques?

OA is A but why C is incorrect?

This is an interesting one! There's a very subtle difference in meaning at work here, and the logic is helpful in getting to the right answer. The sentence talks about scholars doubting two things: Troy's location and its existence.

Troy's location still exists when Renaissance scholars are debating it -- even if the city isn't there anymore, its physical location is (I could take you today to the location of my elementary school, even though it was knocked down several years ago). So we can talk about the location as something that exists in the present, even if it was the site of a past event.

The existence of the city, however, is definitively over, so we can only refer to it as something that happened in the past. Choice C implies that 18th-century historians are in doubt about Troy's current existence, but that's not logical. They are in doubt about whether or not it existed in the past. We use the past perfect to clarify the relationship between the "historians doubted" (recent past, simple past tense) and "Troy had ever existed" (distant past, past perfect tense).

Hope that helps!

Thanks a lot for the detailed explaination.