I'm afraid you ______ go out now. It's too late.

The sentence expresses a prohibition or a lack of permission due to the lateness. Therefore, 'can't' is the correct modal verb to convey this meaning. The sentence implies that going out now is not allowed or advisable. 'Needn't' would suggest it's not necessary, and 'can' would suggest it is permissible, which contradicts the context of it being 'too late'.

Final Answer: can't