The correct answer is B) It was the solution to admit Missouri as a slave state and Maine as a free state.
During 1820, Missouri wanted to enter the United States as a slave state. However, the federal government was hesitant to add Missouri as a slave state, as it would throw off the balance of free state and slave state representation in the House of Representatives and Senate. This balance of free and slave state was important to maintain, as it kept the peace and ensured that no one group was able to gain manipulate federal laws. This is why when Missouri wanted to enter there needed to be a solution.
This is where Maine comes in. Maine will be admitted as a free state to maintain this balance. Along with this, the 36'30 parallel is established. This line states that any territory north of this line that is added into the US from now on will be a free state. Any new territory added below this line will be added as a slave state. This compromise works for about 4 decades, until the Mexican American War ends.
