Final answer:
European slave traders formed alliances with African rulers, who were heavily involved in capturing and selling their people to fill the demand for slaves created by European traders along the West African coast.
Step-by-step explanation:
European slave traders set up posts along the West African coast to form an alliance with African rulers, who raided the interior for captives to sell. European nations such as Portugal, along with others that followed, established these posts to conduct trade in gold, spices, and eventually humans.
The European demand for enslaved captives led to a reorientation of the existing African slave trade, from trans-Saharan to trans-Atlantic, as African political and economic elites captured, transported, and sold Africans to European traders on the coast. These elites frequently launched wars or conducted raids to acquire captives to meet the European demand for human labor.
The Portuguese, for instance, established trade networks and negotiated with African kingdoms, building fortresses known as feitorias to facilitate this trade. Over time, kingdoms within West Africa, such as Whydah and Dahomey, became heavily involved, participating in the capture and sale of Africans to the Europeans in exchange for goods, which militarized these states and reoriented trade towards the slave trade.
In answering the provided multiple-choice question, the correct answer is 'A. African rulers', as they engaged with the Europeans to supply captives for the burgeoning trans-Atlantic slave trade.