Concept:Democracy was ensured by limiting the king's power through traditional councils.Explanation:In the pre-colonial Yoruba political system, the Oba (king) did not rule absolutely.His authority was balanced by councils such as the Oyomesi and the Ogboni.These councils checked the king's decisions and could even advise against abuse of power.This arrangement prevented tyranny and gave the people a voice through their representatives.That is why the key principle that ensured democracy was checks and balances.Answer:A. Checks and balances.