Most populous African city.

**The reasoning** When we talk about the **most populous city**, we're looking at current urban population figures, not just city proper but the entire metropolitan area where people actually live and work. Lagos, Nigeria's commercial capital, has an estimated metropolitan population of over **14 million people** (some estimates push it to 15-21 million). This makes it not only Nigeria's largest city but **Africa's most populous city**. Its rapid urbanization, economic opportunities, and role as West Africa's commercial hub have driven this massive population growth. Cairo comes close with around 10-12 million in the city proper (though Greater Cairo can reach similar numbers to Lagos), but Lagos holds the top spot. **Why the wrong options tempt you** - **Cairo (A)** seems right because Egypt is huge and Cairo is ancient and famous, but population ≠ fame or history - **Kano (C)** is Nigeria's second-largest city (~4 million), but nowhere near Lagos - **Abuja (D)** is Nigeria's capital, and students often confuse "capital city" with "largest city" — classic trap! **Quick takeaway** Capital doesn't mean biggest — Lagos proves that commercial power and migration create mega-cities, making it Africa's population champion.