Answer:
C. Diminish.
Step-by-step explanation:
An antonym is the opposite meaning of a given word. It is the word that gives the complete opposite meaning of the given word.
The word "enhance" means "to increase, intensify, magnify, to make it more than it already is" etc. So, the antonym of that word will be "to diminish, to reduce, to prevent" etc.
Thus, the correct answer is option C.