"The legislative package known as the Compromise of 1850 postponed the Civil War by a decade. However, like the 1820 Missouri Compromise, the Compromise of 1850 failed to resolve the question of slavery in a meaningful way. Over the course of the 1850s, the inadequacies of both measures were made painfully clear. 'Popular sovereignty' undermined the Missouri compromise by suggesting the earlier division of the country along the thirty-sixth parallel into free states and slave states no longer applied. Indeed, the Kansas-Nebraska Act of 1854 permitted slavery. The resulting bloodshed in Kansas, like later incidents at Harper's Ferry, presaged the violent conflict of the Civil War."
—Library of Congress
Use the passage to answer the question.
What does the author mean in saying that popular sovereignty "undermined the Missouri compromise"?
It maintained the division of free and slave states.
It expanded slavery to the Northern free states.
It forced slaves who had escaped to be returned to a slave state.
It reopened debate over the border between free and slave states. (my answer)
Well, it seems the author is suggesting that popular sovereignty undermined the Missouri Compromise by reopening the debate over the border between free and slave states. You see, popular sovereignty allowed the residents of a territory to decide for themselves whether to permit slavery or not. This meant that the previously established division of free and slave states along the thirty-sixth parallel was no longer as clear-cut as before. So, let's say the author is saying that popular sovereignty threw a bit of a clown wrench into the interpretation of the Missouri Compromise.
Your answer, "It reopened debate over the border between free and slave states," is correct.
The passage explains that popular sovereignty undermined the Missouri Compromise by suggesting that the division of the country along the thirty-sixth parallel into free states and slave states was no longer applicable. Popular sovereignty allowed people in newly acquired territories to decide for themselves whether to permit slavery, without regard to the previously established division. This reopening of the slavery debate and disregard for the Missouri Compromise's boundaries ultimately led to the expansion of slavery into territories that were supposed to be free, such as with the Kansas-Nebraska Act of 1854.