Concept:Nigeria's most noticeable role in the African Union is linked to a Nigerian leader who held a top AU position.
Explanation:The African Union (AU) was established in 2002 as the successor to the Organisation of African Unity.
President Olusegun Obasanjo served as the Chairman of the AU from 2004 to 2006.
During this period, Nigeria also played a leading role in the AU Peace and Security Council.
Obasanjo actively represented the AU on major continental peace missions and conflict resolution efforts.
He also championed the African Peer Review Mechanism and hosted AU summits in Nigeria.
No other leader listed in the options held such a prominent AU leadership position during his regime.
This makes Obasanjo's tenure the most prominent for Nigeria's involvement in the AU.
Answer:A. President Olusegun Obasanjo