Final answer:
The Kansas-Nebraska Act of 1854 established Kansas and Nebraska as territories and employed popular sovereignty to decide on the legality of slavery within them. It led to increased sectional conflict and was key in forming the Republican Party.
Step-by-step explanation:
The Kansas-Nebraska Act was a significant legislative act passed by Congress in 1854, which is best described by option A: it established the territories of Kansas and Nebraska and allowed the settlers there to decide through popular sovereignty whether to allow slavery or not. This act effectively repealed the Missouri Compromise by allowing these territories to choose the legality of slavery through popular vote, rather than being determined by a geographical boundary.
Senator Stephen A. Douglas from Illinois introduced the bill with multiple aims, including promoting Democratic party unity, accommodating southern interests by potentially expanding slavery, and facilitating the construction of a transcontinental railroad through Chicago.
The implementation of popular sovereignty led to significant conflict and violence in what became known as "Bleeding Kansas," as pro-slavery and anti-slavery settlers rushed into the Kansas Territory to influence the vote. The Kansas-Nebraska Act had the unintended consequence of exacerbating sectional tensions and contributing to the collapse of the Second Party System as it led to the formation of the Republican Party, composed of northerners who opposed the expansion of slavery.