The event that led most directly to secession was
(a) harper's ferry
(b) the panic of 1857
(c) lincoln's election
(d) the dred scott decision

I would argue that C Lincoln's election led most directly to the secession of Southern states. The 1860 election resulted in the victory of Abraham Lincoln despite his poor showing (and in some states, not even appearing) on the ballot. Lincoln represented a hard-line against Southern slavery and thus served as a sort of an accumulation of other events such as Harper's Ferry and the Dred Scott decision. However, many historians have spilled gallons of ink writing in favor or in opposition of my opinion!