The correct choice to complete the sentence is (Choice C) "was."
So the completed sentence would read: "Occupying a significant part of modern-day Nigeria, the Kingdom of Benin was one of the major powers in West Africa between the thirteenth and nineteenth centuries. It was ruled by Oba Ewuare I from 1440 to 1473."
Using "was" is appropriate here as it refers to a completed past action or state.