The correct answer is North of 36'30 parallel.
The Missouri Compromise (1820) was one of the most significant pre-Civil War compromises. This is due to the fact that the Missouri Compromise outlawed slavery north of the 36'30 line. This meant that new territories added by the United States during this era of westward expansion that were north of this line would be free.
However, this line would become controversial as the United States added territory on the west coast, including California.