Under the compromise, became the northern boundary for
future slave states.
O the Great Plains
O the Mississippi River
O Missouri's southern boundary
O New Mexico's northern boundary

Missouri's southern Boundary

Explanation:

After the Missouri Compromise was signed by both the CSA and the Union, the law stated that every state that formed underneath Missouri's border was a part of the CSA. It also stated that any state that formed above Missouri's Southern border was a part of the Union. But the Union broke the deal and California became a Union state.

Missouri's southern boundary

Explanation:

Under the compromise,Missouri's southern boundary became the northern boundary for future slave states.