IMO it is B.
Premise: A newly discovered disease is thought to be caused by a certain bacterium
Premise: However, recently released data notes that the bacterium thrives(prosper) in the presence of a certain virus
Conclusion: implying that it is actually the virus that causes the new disease
We need to find the supportive evidence.

(A) In the absence of the virus, the disease has been observed to follow infection by the bacterium.
>> This is not supporting statement. It talks about after disease scenario.
(B) The virus has been shown to aid the growth of bacterium, a process which often leads to the onset of the disease.
>> Yes. This supports the conclusion.
(C) The virus alone has been observed in many cases of the disease.
>> This is conflicting. If this is correct then the first premise will go wrong.
(D) In cases where the disease does not develop, infection by the bacterium is usually preceded by infection by the virus.
>> This shows infection by bacteruim is preceded by infection by virus, but specifies in cases where disease does not develop. We should be looking for the disease cases.
(E) Onset of the disease usually follows infection by both the virus and the bacterium.
>> Talking about aftermath of disease. Not useful.
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+1Kudos, if this helps

OA please?
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C; We need to establish virus as the cause of the disease; there are two conflicting premises - and we are supposed to strengthen one (the data premise) over the other.

C is my take. We need to support Virus. Virus alone causes the disease along with promoting the bacteria.
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Finally KISSedGMAT 700 times 450 to 700 An exprience

I think it's C, we need show that the only cause of the disease is the Virus not both
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Nanquam Fractum

I will go for B

I vote for C too. What's the OA?

B ...........boys

The argument: a new disease is discovered AND it is helped by the presence of certain virus, to the onset of the disease

B..............
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KUDOS is the good manner to help the entire community.

OA is C. I picked this by google search.

B is too much of a stretch/extreme

Let's focus on the question first because that is all that is relevant to us. (In fact, it helps to read the question first so that you can read the argument with the question in perspective)
Which of the following pieces of evidence would most support the data’s implication?

Now, before we move on, what is data's implication?
implying that it is actually the virus that causes the new disease.

Now you have to pick an answer choice which best supports 'it is actually the virus that causes the disease'. Focus on that.

Option (C) clearly states that the virus alone has been found in many cases. Then it must be the virus that causes the disease.
It does not conflict with the premise above - 'A newly discovered disease is thought to be caused by a certain bacterium.' The premise does not say that it is caused by the bacterium. It only says that it is thought to be caused by the bacterium.

Option (B) doesn't strengthen that it is actually the virus that causes the disease. It says that the virus aids the growth of bacterium and this growth leads to development of the disease. The cause of the disease is still the bacterium. That is, if the bacterium weren't present, the virus probably couldn't do anything alone. (Note the probably here. This statement doesn't say that the virus alone cannot do anything but it also doesn't say that the virus alone can cause the disease) But we want to strengthen that it is actually the virus which is the cause.
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