Concept:A city that serves as both a national capital and a harbour or seaport.
Explanation:Accra is the capital of Ghana, but the country's main harbour is at Tema, not Accra.
Abidjan is a major harbour city in Côte d'Ivoire, but the official capital is Yamoussoukro.
Abuja is the capital of Nigeria, yet it lies inland and is not a harbour city.
Banjul is the capital of The Gambia, and it is located on an island at the mouth of the Gambia River, functioning as a port and harbour.
Therefore, Banjul satisfies both conditions.
Answer:C. Banjul