What legislation did Stephen Douglas author that created the Kansas and Nebraska territories?

The Kansas-Nebraska Act is indeed the correct answer because it was introduced by Stephen Douglas in 1854 to create the territories of Kansas and Nebraska. The act aimed to facilitate the construction of a transcontinental railroad, which Douglas believed was essential for national growth and progress. Importantly, the Kansas-Nebraska Act also allowed the settlers in those territories to determine for themselves whether they would allow slavery, a principle known as popular sovereignty. This fundamentally altered the political landscape in the United States, reigniting debates over slavery and leading to violent conflicts known as "Bleeding Kansas."

Other pieces of legislation mentioned served different purposes. The Compromise of 1850 addressed various issues related to slavery in the territories acquired during the Mexican-American War but did not create the Kansas and Nebraska territories. The Missouri Compromise, enacted in 1820, aimed to maintain a balance between free and slave states but was rendered partly obsolete by the Kansas-Nebraska Act. Lastly, the Wilmot Proviso proposed prohibiting slavery in any territory acquired from Mexico but was never passed into law.