The answer is letter c. 1863. It was an important event in the Civil War because it was an important stage in putting an end to slavery. During this period, Lincoln policies no shifted to outlawing slavery both in the North and South. There were reasons why it took time for them to formally issue this Proclamation. If it was issued prematurely, it might cause some of the states who had slaves but were still part of the Union. Apart from that, they needed a military victory before it was proclaimed.