What causes the tail of a comet to become visible?

A. Earth

B. Moon

C. Sun

D. Asteroids

Well I feel like these answers are very shallow and need to be more specific. However I would say that this would probably be A or D because two things can make a asteroids tail visible which are if the comet comes in contact with earths atmosphere and that the comets tail is mainly made of dust when it is visible. Overall I would say that it is A but I am not entirely sure ...

The answer of this question is c