In 1862, congress passed a law that stated
a. african americans had to join the military to gain their freedom.
b. african americans could enlist in the military.
c. freed african americans had to serve in the military.
d. african americans could not enlist in the military.

In 1862, Congress passed a law that stated that

B. African Americans COULD enlist in the military. 

This is because if African American soldiers were captured by the Confederates, they were treated more harshly then if it were a "white" Union soldier.

The African Americans had the choice to join or not.

hope this helps :D