Final answer:
Option (C), The election of Abraham Lincoln set off the secessionists, leading to the formation of the Confederate States of America.
Step-by-step explanation:
The election that set off the secessionists was the election of Abraham Lincoln. The 1860 presidential election was marked by deep divisions over the issue of slavery, with the Democratic Party splitting into three factions. The Republicans seized this opportunity and Abraham Lincoln won the presidency, an event that prompted South Carolina and then six other states from the Lower South to secede from the Union and eventually form the Confederate States of America.
While the Dred Scott decision and the Lincoln-Douglas debates greatly contributed to the political tension, it was Lincoln's election that was the immediate cause for secession.