A newly discovered disease is thought to be caused by a certain bacter

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.