They held Political Prisoners.
The transatlantic slave trade was really a triangle. Here's a map to show the approximate routes. The trade was brought about to benefit the economy of Caribbean nations and Europe. The slaves were forced into awful conditions; cramped boats and harsh climates with which to toil away picking cotton or whatnot.
I would think it would be D.
A is incorrect as slavery was just made illegal.
B is incorrect as the United States could not allowed African Americans to not abide by laws.
C is incorrect as it encourage the slaves to assault the slave owners