Concept:The presidential system in Nigeria was first introduced by a specific constitution that replaced the parliamentary system.Explanation:Before 1979, Nigeria used a parliamentary system under the 1963 Constitution.In that arrangement, the Head of State and the Head of Government were separate positions.The 1979 Constitution adopted the American-style presidential system.Under this system, executive power is concentrated in a single president.Shehu Shagari became the first executive president under this constitution.Answer:C. 1979 Constitution