What question caused tensions between the U.S. federal government and the South Carolina state government during the Nullification Crisis?

A) Is a state required to abide by a federal law that the state believes in unconstitutional?
B) Can Congress regulate the practice of slavery in territories of the United States?
C) Do state governors or the U.S. president have ultimate authority over state militias?
D) Are citizens of states that have outlawed slavery required to abide by the Fugitive Slave Act?