Concept:The 1979 Constitution introduced an executive presidential system in Nigeria.
Explanation:Before 1979, Nigeria used a parliamentary system with a Prime Minister as head of government.
The 1979 Constitution changed this arrangement.
It created an executive President.
Under this system, the President is both the Head of State and the Head of Government.
The President exercises executive powers and is responsible for running the government.
The Speaker of the House of Representatives and the Senate President are legislative officers, not heads of government.
Thus, the head of government under the 1979 Constitution is the President.
Answer:A. president