Answer:
D) The Southern states since they employed so many slaves on their plantations.
Explanation:
At the signing of the Constitution, most of the slaves in the U.S. were found in the South, working on plantations. The number of delegates a state gets is dependent on its population, so it is good for a state to have a large population count. The South benefitted from the 3/5 compromise because it boosted their population numbers, giving them more delegates (more delegates mean they control more of the government, which states want).