Which act specifically required the return of runaway slaves to their owners?

The correct answer is the Fugitive Slave Act. This act, passed in 1850 as part of the Compromise of 1850, specifically mandated that runaway slaves who escaped to free states be returned to their owners in the slaveholding states. The Fugitive Slave Act heightened tensions between the North and the South, as it enforced a legal obligation on citizens to assist in the capture of fugitive slaves, which many in the North resisted. It also allowed slave owners to retrieve their slaves without a trial or juries, which many perceived as a violation of their rights and personal liberties.

Choosing the Compromise of 1850 alone does not specifically denote the requirement for the return of runaway slaves, as it contained several provisions addressing various issues, including the balance between free and slave states, and other territorial matters. The Kansas-Nebraska Act pertained more to the expansion of slavery into new territories and the principle of popular sovereignty, while the Missouri Compromise focused on maintaining the balance between free and slave states. Thus, the specificity of the requirement to return runaway slaves is unique to the Fugitive Slave Act.