Answer:
The BEST reason why southern states seceded after the election of 1860 is C) Lincoln promised to abolish slavery in every state as soon as he took office.
Lincoln's victory in the presidential election of 1860 was a major factor leading to the secession of southern states from the United States. Lincoln was opposed to the expansion of slavery into new territories, and southern states believed that his election signaled a threat to their way of life and economic system based on slavery. Many southern states felt that they could not coexist with a president who was opposed to slavery and who they believed would use the power of the federal government to interfere with their way of life.
Therefore, Lincoln's promise to abolish slavery in every state as soon as he took office was a key reason why southern states seceded from the Union, leading to the start of the American Civil War in 1861.