The correct response is:
C. Vicksburg's location along the Mississippi River made it a key city for Confederate forces.
The Battle of Vicksburg, which concluded on July 4, 1863, was crucial because it gave the Union control over the Mississippi River, effectively splitting the Confederacy in two and cutting off supply lines between the Eastern and Western Confederate states. This was a strategic victory for the Union, while it was a significant loss for the Confederates, impacting their ability to sustain the war effort.