The Missouri Compromise of 1820 primarily addressed the issue of slavery in new states and territories. Specifically, it made slavery illegal in new states north of the 36th parallel (except for Missouri). Therefore, the correct answer is:
It made slavery illegal in new states north of the 36th parallel.
The compromise helped to maintain a balance between slave and free states in the Union, but it was ultimately a temporary solution to the larger conflict over slavery in the United States.