The correct answer is Maine.
The Compromise of 1820 (also known as the Missouri Compromise) was an important piece of federal legislation. As the United States was expanding, the issue of whether or not slavery would exist in these new territories became a problem in Congress. In order to keep the peace, Henry Clay helped to develop the Missouri Compromise.
This would allow Missouri to enter the Union as a slave state and Maine to enter as a free state. This would ensure that there would still be a balance of free and slave state representatives in Congress. Along with this, the compromise established the 36'30 line, in which slavery would only exist in the territories south of this border. This would help to delay the Civil War and would be used for roughly 4 decades in the US.