Question

what argument did senators from the south make in favor of admitting missouri as a state?
they felt that new states should be free to decide whether they wanted enslavement or not.
they wanted the balance of power to shift toward free states to slowly phase out enslavement.
they felt that free states should be banned in lands north of 36 degrees.
they pointed out that maine was already becoming a free state.

Explanation:

During the Missouri Compromise debates, Southern senators argued that new states should have the right to decide their own slavery status (popular sovereignty in this context) to protect their economic and social systems dependent on slavery. The other options are incorrect: Southerners opposed shifting power to free states, the 36°30' line was a compromise measure not their core pro-admission argument, and Maine's status was a counterpoint for balance, not their main favor argument.

Answer:

A. They felt that new states should be free to decide whether they wanted enslavement or not.