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

Solution(By Examveda Team)

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.