The answer is C, it would give southern states greater representation in Congress.
Remember that enslaved people could not vote, and so A is incorrect. Enslaved people were property, and did not have the basic right of being able to vote.
B is incorrect because opposing slavery would not necessarily allow the institution of slavery to be preserved.
D is incorrect because northern states wanted slaves to be free, while the southern states didn’t. If slaves were free they would have to be given the right to vote, but that doesn’t really relate to the question in this context. It also doesn’t relate to why they shouldn’t have or should have been included in state populations.