The correct answer is option 'B' - might.

Explanation:
In this sentence, we are talking about the possibility of someone coming tomorrow. Let's analyze the given options and see why the answer is 'might':

a) shall: 'Shall' is used to express a future action or possibility, but it implies a strong intention or determination. It doesn't convey the sense of possibility that we need in this sentence.

b) might: 'Might' is used to express possibility or probability. It suggests that there is a chance for someone to come tomorrow, but it's not certain. This fits the context of the sentence.

c) must: 'Must' is used to express obligation or necessity. It indicates a strong certainty, which doesn't align with the sense of possibility in the sentence.

d) can: 'Can' is used to express ability or permission. While it could indicate the ability to come tomorrow, it doesn't convey the sense of possibility that we need here.

Therefore, the correct answer is 'might' as it suggests the possibility of someone coming tomorrow without implying certainty.