Concept:The key difference lies in the system of government adopted by each constitution. The 1963 Constitution retained a parliamentary system, while the 1979 Constitution introduced an executive presidential system.
Explanation:Under the 1963 Constitution, Nigeria operated a parliamentary system based on the Westminster model. In that system, the Prime Minister was the head of government, and the President was only the ceremonial head of state.
The 1979 Constitution was created after the civil war to correct the weaknesses of the previous system. It adopted the American-style presidential system of government.
In this new system, the President became both the head of state and the head of government. The President exercised real executive powers and was directly elected. This change clearly separated the executive from the legislature.
Republicanism existed before 1979, and Nigeria remained a federation, so those options are not the distinguishing feature.
Answer:C. presidential system of government.