The correct answer to this is C. The main port of the Kingdom of Aksum (sometimes spelled “Axum”) was a city called Adulis. This city was in what is now the country of Eritrea.
The Kingdom of Aksum existed from about 100 to about 940 AD. It was located in the area around the Red Sea, mostly in what is now Eritrea and the northern part of Ethiopia. Aksum became very important for trade between the Roman Empire (through Egypt) and India. Around 100 BC, this trade came to be conducted through ports on the Red Sea. This put Aksum in a good position. This was a major reason why Adulis became such an important city.