People contract viral infections more often during the winter than at

Had to pick between (C) and (E). Ultimately chose (C). It took me 01:50.

(A) "unquestionably" can modify either "contract" or "because ..." clause. Such ambiguity should be avoided.
"because, for one reason” is redundant.

(B) No meaningful antecedent for "it".
The use of "if clause" leaves the meaning rather obscure. We do not even know, for a fact, that "people contract viral infections more during the winter than other seasons".

(C) Despite my ultimate choice of (C), I am cognizant of some issues with this version
<1> "more people" or simply "more"? In (C), more can be interpreted to be "more frequently". It is not exactly the same meaning as "more people". But any reasonable meaning should be accepted as we are not privy to the writer's intension.
<2> "than at other times" or "than other times"? For perfect parallelism, "than at other times" is better. Written this way, (C) may be misinterpreted, however unlikely, to mean "people contract viral infections more during the winter than other times contract viral infections". But I think "other times" itself can be used adverbially. For example: I am morose every spring; I feel much happier other times of the year.
Examples with ”during the winter than other times“ from reputable sources abound.
<3> I would like to see "that" after "is".
If the correct answer is indeed (C), other options must sport even worse writings. Let's see.

(D) "reason...because" is unquestionably redundant.

(E) We can also argue that "reason why" is redundant. Read more at https://english.stackexchange.com/quest ... 0Online%3A
Some redundancy is not fatal, but what is GMAT's perspective?
Also, when I read the sentence, I did not like "is unquestionably that". One reason that I did not like it is probably that I have never read such collocation. After spending more time writing the reply, well, I do not find it so objectionable.

So, the answer might be (E), but I did choose (C) as my answer.