The correct option is
C May
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 A is the correct answer because it expresses the mood of extreme probability
Whereas option B expresses weak possibility; option D expresses unsure certainty with which certain act is to be done in future; option C expresses ability which was supposed to be shown in the past.