The African state who by sheer coincidence proved to be the greatest contributor to the European slave trade was
By

Although there were large trading centers along the rivers—the Senegal, Gambia, Niger, Volta, and Congo—most West Africans lived in small villages and identified primarily with their extended family or clan, rather than an ethnic or national identity. Wives, children, and dependents were a sign of wealth; men frequently practiced polygyny, or the custom of having more than one wife. In times of need, West Africans relied on relatives from near and far for support. Hundreds of separate dialects emerged from different west African clans; in modern Nigeria, nearly 500 languages are still spoken.

d. promised it other land

because it took place after the war of 1812 had come to an end.