If you just think about the geography of the world, you will be able to think of this answer for yourself.
First, which area of the world was importing slaves? As you know, most of the slaves were being taken from Africa to the Americas (mostly the Caribbean and Brazil).
Given that, which part of Africa would be most likely to be affected? Of course it will be the part of Africa that is nearest to the Americas.
Because of this, the correct answer here is B -- West Africa was most impacted by the slave trade.