What series of agreements was passed by Congress in 1820-1821 to maintain the balance of power between slave and free states?

The Missouri Compromise was a significant legislative agreement enacted in 1820-1821 to address the contentious issue of slavery in newly acquired territories and states. This agreement was crucial in maintaining a fragile balance between the number of free states and slave states in the Union. Specifically, it allowed Missouri to enter the Union as a slave state while simultaneously admitting Maine as a free state, thus preserving the balance in the Senate.

Additionally, the Missouri Compromise established a geographic boundary at the southern border of Missouri, prohibiting slavery in the territories north of that line (except for Missouri itself). This compromise temporarily alleviated sectional tensions between the North and South, which were already strong by that time due to the expansion of slavery into new territories.

The other options represent different historical contexts and legislative actions regarding slavery but did not address the immediate needs for balance in the early 1820s the way the Missouri Compromise did.