The correct option is A would cost Douglas his Senate seat in the 1858 race.
Stephen Douglas's Freeport Doctrine came after the failure of his Kansas Nebraska Act, which though passed lost the support both from the North and the South. The North opposed the spread of slavery and the south wanted it to extend. According to Douglas's Freeport Doctrine, the settlers couldn't prohibit slavery in the territory in the pre-state or pre-constitution state.