Let's take a look at this like this:
Abraham Lincoln ran for the president twice, in 1860 and in 1864. The Civil War started only in 1861, so the relevant election is the one in 1864.
This election had two candidates: Abraham Lincoln and George B. McClellan. (Additionally John C. Fremont was running in this election in the early stages, but later withdrew).
Therefore the only possible answer is C. McClellan
