Answer: C. Slavery
Step-by-step explanation:
Even though there were several reasons for the American Civil War, Slavery was the most prominent and controversial. The Federal government was increasingly being controlled by the anti-slavery Republican party which culminated with the Republican President, Abraham Lincoln being elected.
The Southern State government did not appreciate the measures being taken by the federal government to limit slavery and saw this as an attack on their way of life and so seceded from the U.S., an action which precipitated all out war.