Answer: B. the spread of slavery into the West.
Explanation:
It was an established fact in the years leading up to the Civil War that the South was pro-slavery and the North was Anti-slavery and so both sides had arguments about the spread of slavery into the western territories as those territories gained statehood.
The South wanted slavery to spread into the territories so that they would have more states which would give slavery more representation in Congress and therefore keep the practice going. The North wanted the territories to be free for the same reason.