The answer is C, “Lincoln promises to restore peace”.
In that period of time, the desired peace hadn’t been achieved yet so, after the catastrophic events that occurred because of the war (dead soldiers and as a result of that, lonely families), he tries to set calm in society by promising to restore the peace. What supports the answer is noticeable when it says “let us strive to finish the work we are in, to bind up the nation’s wounds, to care for him whom shall have borne the battle…”. He wants to finish the work they are doing so that they restore the nation’s peace.