AU headquarters.

**The reasoning** The African Union (AU) is the continental body uniting all African nations, and its headquarters is in **Addis Ababa, Ethiopia**. This was established when the Organization of African Unity (OAU) transformed into the AU in 2002. Ethiopia was chosen partly because it's one of Africa's oldest independent nations (never colonized except for a brief Italian occupation) and hosts a long diplomatic tradition. The AU headquarters building, gifted by China, stands as a symbol of African unity. **Why the wrong options tempt you** - **Abuja (A)** might catch you because it's Nigeria's capital and ECOWAS headquarters, making you think "major African organization = Nigerian city." - **Accra (B)** could tempt you since Ghana was first to gain independence and has diplomatic significance. - **Cairo (D)** seems plausible as Egypt is a major African power and historically significant, plus it housed the Arab League headquarters. **Quick takeaway** Think **"AU = Addis"** — both start with 'A', and Ethiopia's independence history made it the natural home for Africa's unity organization.