Final answer:
The Kansas-Nebraska Act overturned a previous agreement on the status of slavery in western territories by allowing territories to decide through popular sovereignty, leading to increased sectional tensions and conflicts.
Step-by-step explanation:
The legislation that overturned a previous agreement regarding the status of slavery in the western territories of the United States is d) the Kansas-Nebraska Act. The Kansas-Nebraska Act was introduced by Senator Stephen Douglas in 1854 and effectively repealed the Missouri Compromise by allowing the territories of Kansas and Nebraska to decide the issue of slavery through the concept of popular sovereignty.
This act heightened sectional tensions since it opened up the possibility for slavery to spread into areas that had been free under the Missouri Compromise, contributing to violent conflicts in Kansas known as 'Bleeding Kansas' and sowing the seeds for the American Civil War. By promoting popular sovereignty, the act gave residents of the territories the right to vote on whether they would be free or slave states, which led to both proslavery and antislavery forces attempting to influence the decision, often by violent means.