The correct answer is D) whips.
Whips assist the floor leaders in getting party members to vote a certain way.
We are talking about a function in the United States Congress. Whips are the assistants of the floor leaders. They have to be elected by the party conferences. Whips have assistants that have the responsibility of assembling votes with the members of their parties when the issue is of utmost importance. The whip cal also serves as a floor leader when in need.