B) gold and salt. is the answer
because At the height of its power, the Mali Empire used to control the trade of two important natural elements such as gold and salt. The Mali Empire had routes throughout the Sahara Desert to the coasts of North Africa. Mali Empire used camels and caravans to transport the products. Other nations were marveled by the gold but also were in need of salt for the preservation of their food.
Hopes this helps!
I read something like this in history class.