Final answer:
The Kansas-Nebraska Act revolved around the expansion of slavery into new territories, allowing the settlers to determine whether they would permit slavery, leading to increased national tensions and violence in Kansas. The correct answer is option 3.
Step-by-step explanation:
The Kansas-Nebraska Act of 1854 was a pivotal piece of legislation that concerned the expansion of slavery into new territories. It effectively repealed the Missouri Compromise line by applying the concept of popular sovereignty to the territories, meaning that the settlers themselves could decide whether or not to allow slavery. This led to a violent struggle known as "Bleeding Kansas," as pro- and anti-slavery forces fought over control of the future state's constitution and status regarding slavery. The act also contributed significantly to the rise of the Republican Party and increased tensions that eventually led to the Civil War.