Final answer:
The Swahili city-states, especially Kilwa, were primarily enriched by the trade industry, with gold trade from Sofala being a key contributor to their economies.
Step-by-step explanation:
The industry that contributed most to the economy of the African city-states of Kilwa, Malindi, Mombasa, and Sofala was trade, specifically in gold, ivory, furs, and enslaved Africans. Kilwa, in particular, established its prominence through control of the gold trade from Sofala, which it leveraged to gain dominance over other city-states. The wealth generated from this trade allowed these Swahili city-states to grow and prosper, with Kilwa becoming the most important and wealthiest among them, aided by its strategic position to exploit the monsoon winds and participate in the far-reaching Indian Ocean trade network.