Which part of the fugitive slave act was the north most upset with?

California would enter the union as a free state
The slave trade would be abolished in Washington DC
Texas and New Mexico would solve their border disputes
Slaves escaping to the north needed to be returned to the south

The part of the Fugitive Slave Act that the North was most upset with was the provision that slaves escaping to the north needed to be returned to the South. This sparked outrage and resistance in many Northern states, leading to the formation of personal liberty laws and resistance to the enforcement of the act.