The correct answer is: Representation in Congress.
Step-by-step explanation:
The Southern states based their economy on slavery while the Northern states were free states, with bigger population due to the industrialization so their Representation in Congress was a big issue.
Debates of Missouri being admitted as a part of the Union after the Louisiana's Purchase generated two different opinions. That Missouri should be admitted as a free state because the Northern states were free states and were against slavery and that Missouri should be admitted as a slave state because every state should be able to make their own laws and decide if they wanted to have slaves or not.
Southern states supported Missouri's decision of being a slave state, so they argued that The Three-Fifths Compromise stated that 3 out of every 5 slaves were to be counted in order to establish representation in Congress, so if they were admitted as a slave state they would have less representation in Congress than they would have being a free state. Southern states also argued that African Americans in Northern states had different rights they couldn't join the army or enter to free public places, so it wouldn't make a big difference if Missouri was a free state or a slave state. Missouri was admitted as a slave state in exchange of Maine being admitted as a free state.