The phrase that popular sovereignty "undermined the Missouri Compromise" refers to the fact that popular sovereignty, which allowed settlers in a territory to decide for themselves whether to allow slavery, reopened the debate over whether new states would be free or slave states. This was contrary to the Missouri Compromise, which had established a clear line of demarcation between free and slave territories.
Therefore, the correct answer is:
- It reopened debate over the border between free and slave states.