The conclusion: Keratitis in mice infected with a herpesvirus is caused by antibodies to herpesvirus.
The evidence: The immune systems of mice typically produce antibodies that destroy the virus by binding to proteins on its surface. Proteins on the surface of cells in the eye closely resemble those on the herpesvirus surface. Mice infected with a herpesvirus generally develop keratitis, an eye disease.

You are asked to support the conclusion/hypothesis. We are told that mice that produce the herpes antibodies get keratitis, the eye disease. Choice D, the correct choice, states that mice that DO NOT produce antibodies in response to herpes DO NOT get keratitis. This strongly supports the claim that the antibodies cause the keratitis.

Choice A is an "other" choice. We are not concerned with other types of viruses.
Choice B refers tells us that mice that cannot form antibodies to herpes get herpes at about the same rate as those mice that can form the antibodies. This is not a relevant comparison. What about the keratitis?
Choice C actually weakens the conclusion. If some mice don't get keratitis even though they produce antibodies, then it seems that the antibodies are not causing keratitis.
Choice E also weakens the conclusion. If a mouse does not get herpes, then it won't get the herpes antibodies. This choice says that these mice, without herpes, still can get keratitis. Therefore, the antibodies didn't cause the keratitis.

I'm available if you'd like a follow-up.