Which of the following did congress attempt in the mid-1800s?
a. establishing a balance of free and slave states
b. creating more free states than slave states
c. creating more slave states than free states
d. forming new political parties to address slavery

A. Establishing a balance of free and slave states

Congress attempted to do this with the Missouri Compromise (1920) which stated that for every slave state, there must be one free state. The Compromise was the result of long debates in Congress over the issue of whether to admit Missouri to the Union as a slave state or not. At the time, the United States had 22 states, 11 were free and 11 were slaves. The admission of Missouri as a slave state would shift the balance, it would mean that Congress would support more the expansion of slavery, and that would've angered the abolitionists.

The Missouri Compromise was the solution to the conflict. It admitted Missouri as a slave state and Maine, which was originally part of Massachusetts, as a free state; and stated that slavery was to be excluded from the Louisiana Purchase lands north of latitude 36°30′ (except for Missouri).