Explanation

Reading the question: We have seen a fill-in-the-blank question before, in "Auto Body." Such questions may be phrased with or without a printed line signifying the blank. As we discussed in Auto Body, what goes in the blank should be a logical continuation of the prompt. Logical tests tend not to be possible on this type of question, so we will rely on our filter, ideally a prediction.

Creating a filter: how can we predict what goes in the blank? It must not contradict what has come before and it should finish the expression of the idea. The key is that the virus is on the computer even though the scanning software hasn't detected it. We could imagine that there is a defect in the scanner, or an ingenious design to the virus. Note that you are not expected to know anything about IT security for this question. Critical Reasoning questions are designed not to require any outside knowledge. Outside knowledge may still be useful, inevitably, and that's why the questions cover a range of topics. We could decide on the prediction, "The virus has outsmarted the security systems," as our filter.

Applying the filter: Do any answer choices match our prediction? Choice (A) somewhat does, but rather vaguely. Choice (B) doesn't address why the virus wasn't detected. Choice (C) also doesn't seem to address why the virus wasn't detected, though we might be able to read something into the term "backdoor." Choice (D) would definitely constitute outsmarting the security system. Choice (E) involves outsmarting, but not in a way that would necessarily obscure detection. We're left with (D). We confirm the logic: the correct answer must give an explanation for how a virus might have gone undetected by security systems. Choice (D) is the only option. Choice (C) sounds close, but we presume that many viruses possess backdoors and are detected. The correct answer is (D).