Answer: B. The North Only
Step-by-step explanation:
This question seems like it was made to trick you. Much of the western US was anti-Slavery prior to the Civil War, but the US didn't acquire much of its western territory until after the signing of the Treaty of Guadalupe Hidalgo in 1848. The US extended westward some prior to that, but not much, and the area that was settled by the US prior to the war was mostly split between being slave or free states. Therefore, the best answer to this question, because of the time frame, would be The North Only.