Final answer:
The Confederacy's key advantage in the Civil War was the ability to conduct a defensive war within their own territory, with knowledge of the terrain and civilian support, coupled with skilled military leadership and a populace experienced in firearms and horseback riding.
Step-by-step explanation:
The statement that best describes one advantage that the Confederacy had over the Union in the Civil War is the Confederacy's ability to wage a defensive war. The Confederate States were primarily on the defensive, fighting within familiar territory, with support from the local civilian population and the advantage of having to simply outlast the Union forces, rather than defeat them outright. Additionally, the Confederacy had skilled military leaders, with many having attended West Point, and a population accustomed to the use of firearms and horseback riding.