The correct answer is C. The student requested to let him come in.
The student is asking for permission to come in. He is not telling anyone to let him come in, nor is he saying that he might be allowed to come in. He is simply requesting permission.
Option A is incorrect because the student is not telling anyone to let him come in. He is requesting permission.
Option B is incorrect because the student is not saying that he is allowed to come in. He is requesting permission.
Option D is incorrect because the student is not requesting that he might be allowed to come in. He is simply requesting permission.