How were the rulers of the early West African empires able to control the trans-Saharan trade?

Gold Trade and the Mali Empire Trans-Saharan routes began to bypass Audaghost, expanding instead toward the newly opened Bure goldfield. Soso, the southern chiefdom of the Soninke, gained control of Ghana as well as the Malinke, the latter eventually liberated by Sundiata Keita, who founded the Mali empire.

the answer is going to be c

either d or b, i'm not to sure. but my gut tells me it's prob going to be b