Concept:The Old Oyo Empire fell mainly due to military pressure from external invaders.Explanation:The Fulani invasion played a major role in the fall of the Old Oyo Empire.The Fulani Jihad, led by Usman dan Fodio, spread into Yoruba territory.This invasion weakened Oyo's military power and political unity.The loss of key towns, including Ilorin, helped destroy the empire.Answer:D. The Fulani.