"Vested power" is power based on a belief in the lawfulness of enacted rules (laws) and the legitimate right of leaders to exercise authority under such rules, since the person in question in thought to embody this authority.
Take off D; the North had won some battles up to that point. Rip off A; the South would never let go of slavery. Take off B, since the Union had some wins already. You can now clearly see where the answer is.
