The correct answer is D) in the East, on the Atlantic Ocean.
Prior to and during the Civil War, the largest population centers in the United States could be found in the East, on the Atlantic Ocean.
The most inhabited places prior to the Civil War and during the confrontation was the East Coast of the United States, along the Atlantic coast. The most important cities had been founded in the Atlantic coast due to that those were the places where the European colonists had arrived since 1609 when the Jamestown colony in Virginia was founded. That way, cities started to grow form those places inward, displacing many Native American Indian tribes.