Option C. his opposition to expanding slavery.
In context, when he was a candidate he was opposed to the expansion of slavery into any U.S. territory and even proposed a compensated emancipation. This was before the Civil War and before he got elected. His aim was to slowly end slavery through gradual emancipation and voluntary colonization, and not by immediately ending slavery.
It was later on when he was already president that he issued the Emancipation Proclamation (in 1863).
