Concept:Diplomatic titles depend on the type of relationship between countries, and Commonwealth members use special titles for their missions.
Explanation:Nigeria is a member of the Commonwealth of Nations.
Among Commonwealth countries, the head of a foreign mission is given the title "High Commissioner" rather than "Ambassador".
This special title reflects the historical and cooperative bond shared by member nations.
An ambassador is normally sent to a country that is not a member of the Commonwealth.
A charge d'affaires is only a temporary or acting head of mission.
An attache is a junior diplomatic officer with a specific role, not the head of the mission.
Therefore, the representative heading Nigeria's mission in another Commonwealth nation is correctly called a High Commissioner.
Answer:A. high commissioner