The correct answer is - enslaved Africans.
Since most of the Native Americans died, mainly because of the spreading of the diseases that the Europeans brought with them, but their immune system was not able to fight them, the Europeans needed workforce for their plantations, and they found it in Africa.
Most of the tribes in Africa tended to enslave the people from the defeated side in their battles and wars, so when the Europeans came and offered them payment for them, they took advantage of the situation and started to sell their slaves to the Europeans. The Europeans were transporting them to their plantations in the New World where they were using the African slaves as laborers on their land.
