Why is it D)? well its D) because the Confederates had to prove they could win a battle on Union soil (to get Britain and France to join on their side) and the Union had to prove it could stop such an invasion. Antietam/Sharpsburg, a Confederate loss on Union soil, was just such an event.
The battle of Antietam, which was fought on September 17, 1862. Lincoln needed this Union victory to issue the 'Emancipation Proclamation'. Just five days after the battle of Antietam the 16th President of the United States issued the proclamation to set about 500,000 slaves free.
