Answer:
Option A
Step-by-step explanation:
One of the Unions military advantages is that they had a way bigger army than the Confederates. The answer is option A "It already had an army and navy." They're over two million men enlisted in the army during the war between the Confederates and the Union and the Union not only had an army double the size of the Confederates, but they also had a navy. It's not option D because the Confederates were the ones who knew the terrain way better than the Union because the war was taking place on Confederate territory.
Hope this helps.