Prior to exploration, enslaved people in africa were often
captives in war.
stolen from europeans.
foreign criminals.
purchased from europeans.

The correct answer is - captives in war.

The slavery in Africa, as in most of the world, existed for a very long time. Prior to the explorations of the Europeans, the slaves in Africa very the people who were captured in a war. The stronger tribes had lot of slaves from the tribes that they managed to defeat and conquer.

When the Europeans had the need for workers on the farms, they started to buy the African slaves, and it benefited both them and the Africans that were selling the slaves. Unfortunately, that developed into a business, so the stronger African tribes started to capture people and enslave them in order to make profit out of them, so it became a very big business on the Atlantic coast for few centuries.

lincoln declared that slavery should not be allowed to spread into the new states, and of course this angered the south at the time.