The true statement about African American soldiers during the Civil War is: They were used to fill the ranks in regiments left by deceased white soldiers.
While it is also true that many African American soldiers faced challenges such as being given menial jobs and that they did not enlist in large numbers until later in the war, the statement about them filling the ranks in regiments left by deceased white soldiers accurately reflects the situation. Additionally, most African American regiments were led by white officers, not African American officers, although there were some exceptions.