. . . . . . . . I come in, sir?

Modal Verbs show us the attitude of the speaker to what is being said or done. The term "modal" means expressing mood and mood is a way to express the attitude of the speaker.
Option C is the correct answer because it expresses possibility and it is generally used in formal sense of asking a permission with politeness applied.
Whereas option D suggests ability that was supposed to show in the past; option A suggests ability; and option B expresses certainty or assurance with which certain act is to be done in future.