Answer: Islam
Step-by-step explanation:
The old Kingdom of Ghana and the Malian and Songhai empires were all located around the same area at different points in time, the Western part of Africa including Mali and the rest of the Western Sahel.
These Empires became very wealthy from the trade of gold and spice such as salt with Arabian traders and this led to alot of interactions with the latter.
As the Arabs settled and traded with these Empires, they spread the message of The Prophet and Islam spread. Indeed, it was because of the foothold that Islam gained in this kingdom that enabled it to then spread to the rest of Africa.