This correct answer is "popular sovereignty."
This idea of letting territories vote on whether or not they wanted slavery was supposed to be helpful in solving the issue of slavery in the United States. However, this led to terrible violence and bloodshed, especially in Kansas. After Kansas voted to become a slave state, many anti-slavery individuals claimed the election was rigged due voters coming from Missouri to vote in the Kansas election. This resulted in fighting between the two. This period is now known as "Bleeding Kansas." This idea was one of many solutions that failed to solve the issue of slavery in America.