Nigeria was a member of

Nigeria is a member of the Organisation of the Petroleum Exporting Countries (OPEC). Nigeria joined OPEC in July 1971, having become a significant crude oil producer. OPEC is an intergovernmental organisation of oil-exporting nations that coordinates and unifies petroleum policies among its member countries to ensure stable oil markets and fair prices.

The other options are organisations Nigeria does not belong to:

• The Arab League is a regional organisation of Arab states in the Middle East and North Africa. Nigeria is not an Arab country and is not a member.
• The EAC (East African Community) is a regional intergovernmental organisation of East African countries including Kenya, Uganda, Tanzania, Rwanda, Burundi, South Sudan, and the Democratic Republic of Congo. Nigeria is in West Africa and belongs to ECOWAS instead.
• NATO (North Atlantic Treaty Organisation) is a military alliance of North American and European countries. Nigeria is not a member.

Nigeria's OPEC membership reflects its status as one of Africa's largest oil producers and gives it a voice in global petroleum pricing and production decisions.