C. Leaving the Union.
Explanation:
The term "secession" refers to the action of leaving the Union which was taken by the Southern states before the American Civil War.
After the election of Abraham Lincoln in 1860, the South immediately seceded from the Union. Tensions between the Northern states and the Southern states were already high because of the issue of slavery. The North was not for slavery and had free states, while the South was for slavery and had slave states. The South relied on slavery for their income and would fight back against anyone who wanted to take it away.
The Southern states felt threatened by the election of 1860, which elected Abraham Lincoln. They felt like he would try to abolish slavery and obviously did not want anything to do with that, so they left the Union and created the Confederacy.
