The answer is: C. Congress was given the power to ban the slave trade after 1808.
At that time, the southern states faced a heavy pressure from northern states to free the slaves in their territories.
The southern states at that time, stated that freeing the slave directly would resulted in the destruction of their economy. So, they made a compromise that the southern states would be given time to prepare until 1808. After that, Congress would be able to put laws to ban the slave trade in united states.