The correct answer is the growing sectionalism over the issue of slavery.
The Missouri Compromise was a result of tensions between Northern and Southern politicians in Congress. As America was expanding and new territories, the question of whether or not slavery would exist in these new territories became a prominent question. For southerners, they saw slaves as property and thought that the institution of slavery should absolutely be allowed. Northerners, on the other hand, did not want slavery to spread. This was due to their fear of Southern politicians gaining too much power in Congress.
This is where the Missouri Compromise comes in. After tense negotiations, it was decided that Missouri would become a slave state, Maine would become a free state, and that slavery would only exist in new territories below the 36'30 parallel. This compromise would be only the beginning of problems over slavery in the US during the 1800's.
