Final answer:
The Missouri Compromise split the territories of the Louisiana Purchase at 36°30'N, with land north of the line becoming free states and land south of the line becoming slave states.
Step-by-step explanation:
The correct answer is a) Missouri Compromise. In 1820, the Missouri Compromise divided the territories of the Louisiana Purchase at 36°30'N. All land north of this line (except Missouri) would become free states, and all land south of the line would become slave states.
The purpose of this compromise was to establish a balance between free and slave states and prevent future conflicts over the expansion of slavery.