Islam was most popular in sub-Saharan Africa in o the commercial centers of west Africa and the Swahili city-states.
O areas that had previously had contact with Christian missionaries
areas that had forsaken the traditional religions because of famine or plague.
O the thinly populated rural areas.

the commercial centers of west Africa and the Swahili city-states.

Explanation:

the commercial centers of west Africa and the Swahili city-states – this is the correct answer. Islam had a big presence in west Africa, especially on the Swahili coast. As city-states started to blossom and become a trade and commercial center, Islam became more and more present in these places. Part of the wealth of city-states is attributed to Islam as well, as they truly flourished after the conversion because of Muslim merchants. Archeologists found remains of the mosques in these areas.

areas that had previously had contact with Christian missionaries – this is not the right answer. The areas who have contact with Christian missionaries weren't popular among Muslims.

areas that had forsaken the traditional religions because of famine or plague - this is not the correct answer. These areas did not have a big Muslim population.

the thinly populated rural areas – this is not the correct answer. Islam wasn’t popular in rural areas.

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