Final answer:
The Fugitive Slave Act, a part of the Compromise of 1850, was the controversial provision that required the return of runaway slaves to the South and imposed harsh penalties on those who aided in their escape or failed to capture them.
Step-by-step explanation:
The controversial provision of the Compromise of 1850 that required runaway slaves to be returned to their owners in the south under harsh penalties was 1) The Fugitive Slave Act. This act was part of a larger legislative package aimed at addressing the sectional tensions around slavery in the United States. The Fugitive Slave Act included several severe measures, such as empowering federal government to deputize citizens in capturing runaways, penalizing individuals who did not cooperate with slave catchers, and denying alleged runaway slaves the right to a jury trial or the ability to testify in their own defense. It was an attempt to enforce Article IV of the Constitution, which stated that escaped slaves should be returned to their owners, but it ended up causing greater sectional strife and tension between the North and South.