The appropriate modal in this context is "must" because it conveys a strong sense of necessity or obligation. 

The sentence is expressing a high degree of certainty or conviction that the answer is wrong. In this context, "must" is used to indicate a logical deduction or a strong conclusion. It implies that there's a very high probability that the answer is indeed wrong, almost to the point of being a certainty.

- "Can" would not convey the same level of certainty or obligation. It is often used to indicate the possibility or capability of something, not the necessity or strong likelihood.

- "May" is used to indicate permission or possibility, which doesn't fit the context of the sentence.

- "Ought to" suggests a recommendation or moral obligation, which is not appropriate in this context where the sentence is asserting a strong belief that the answer is wrong.

So, "must" is the most suitable modal to convey the intended meaning of strong certainty and necessity in this sentence.