How did Congress attempt to maintain a balance of power between Northern and Southern states in the mid-1800s?
by keeping the number of free and slave states equal
by creating more free states than slave states
by created more slave states than free states
by forming new political parties to address slavery

By keeping the number of free slave states equal. (A)

Explanation:

 If Northerners had more free states than Southerners, Southerners would be angry. If Southerners had more slave states, Northerners would be angry. It needed to be an equal amount of free and slave states. If it wasn't equal, it would lead to civil war. The civil war started over the disagreement of the terms for the compromise.