Answer :

Atheists are people who believe that there is no god.  So it's not C.
Monotheists are people who believe in 1 god.  Mono means 1. So not B.
A theologist is a person who studies theology.  So not A.
Poly means many, or a lot.  And a polytheist has 2 or more gods.  So D would fit your bill.

The term that refers to someone who believes in more than one god is a polytheist, a monotheist believes in one god