Final answer:
City-states on the East African coast arose due to trade opportunities, shared religion and language, and their role as trading depots.
Step-by-step explanation:
The city-states along the East African coast arose due to several factors. First, they took advantage of wind patterns in the Indian Ocean, which allowed them to participate in lucrative trade with merchants from Arabia, Persia, and India. Second, the people of these city-states were united by a shared religion, Islam, and a shared language, Swahili. Lastly, these city-states served as trading depots and ports, enabling them to connect the interior of Africa with the rest of the world.