Final answer:
The Northerners in Congress responded by agreeing to a compromise in which Maine was admitted as a free state to balance the admission of Missouri as a slave state. Therefore, the correct option is C.
Step-by-step explanation:
The response of Northerners in Congress to Missouri's proposal to enter the Union as a slave state was C. To balance the admission of Missouri, they agreed to a compromise in which Maine was admitted as a free state. This compromise, known as the Missouri Compromise, was reached in 1820 and aimed to maintain a balance between free and slave states. Under the compromise, a line was drawn at 36° 30' across the remainder of the Louisiana Purchase lands, with slavery being allowed south of the line and prohibited north of it.