Which act allowed popular sovereignty to decide on slavery in Kansas and Nebraska?

The Kansas-Nebraska Act is pivotal in the context of 19th-century American history as it introduced the concept of popular sovereignty, which allowed the residents of Kansas and Nebraska to decide for themselves whether to allow slavery within their territories. Passed in 1854, this act effectively repealed the Missouri Compromise of 1820, which had previously prohibited slavery in certain northern territories.

By allowing the inhabitants of these territories to vote on the issue of slavery, the Kansas-Nebraska Act aimed to resolve the contentious debate over the expansion of slavery in the West, yet it ultimately led to significant conflict and violence, known as "Bleeding Kansas". This conflict highlighted the deep divisions within the nation regarding slavery and intensified the sectional tensions that would eventually lead to the Civil War.

Other options, while relevant to the broader discussion of slavery and territorial governance, do not pertain specifically to the introduction of popular sovereignty. The Missouri Compromise and the Compromise of 1850 sought to address slavery through regulatory means rather than allowing local decision-making. The Wilmot Proviso, meanwhile, proposed to ban slavery in territories acquired from Mexico, but it did not implement popular sovereignty as a solution.