Answer :

The men's shoe department is on the other side of the store. It is already plural. 
C. Men's
The men's shoe department is on the other end of the store.
It is spelled this way because you are saying it is a men's store. It belongs to the men. I looked it up. It would NOT be mens, because that is not even a word. Men is already plural. It would not be mens' because men is already plural. Hope this helps!!! (: