Concept:The Supreme Military Council was the earliest military body in Nigeria that exercised legislative power.Explanation:After the 1966 military coup, the new military government created the Supreme Military Council as its highest law-making body.This council issued decrees which replaced civilian legislative functions.The Armed Forces Ruling Council and the Provisional Ruling Council came later under subsequent regimes.Thus, the first institution introduced by the military for legislative power was the Supreme Military Council.Answer:A. Supreme Military Council.