IMO A.
What is OA?[/quote]

Actually C is just saying what is already concluded by the doctors. The doctors conclude that the virus causes the daibetes. So C just says it in a different way that without this virus the daibetes cannot be developed.

But the assumption we are looking for is the one which says that the daibetes does not cause this virus to be present in the body. This is exactly what D says and so D is correct.

A is not correct because the passage says nothing about the length of time the virus affects a person and so is out of context!

Can someone explain me why the answer is not B?

Figured out the reason:
D eliminates the possibility of reverse causation and VALIDATES conclusion. C is NOT an valid assumption but is a STRENGTHENER. Lets see how,
Argument says X and Y were found in patients and doctors concluded X -> Y. lets says there are Two valid assumptions:
a) Y does not cause X ( mentioned in D, and negating it, Y->X, INVALIDATES the conclusion)
b) Y cannot be caused by any other element Z ( mentioned in C) i.e. Z does not cause Y. but that point is not debated by argument as argument does not say X is THE ONLY cause of Y. Now see C is strengthener because it eliminates some other possibilities that could cause Y, thus making it more likely OR increasing probability of X -> Y. Also try to negate C and it says "Individuals not infected with virus V CAN develop atypical diabetes." i.e. there can be some other causes for Y. So what conclusion still holds as X can be one of the causes.

B is not correct because Y is a symptom of X tells us that X happened first which might seem correct but SYMPTOM is NOT equal to EFFECT. SYMPTOMS may/may not appear but in CAUSAL relationship EFFECT is bound to appear.