The answer would be D. boats
Even though a lot of traders from outside Africa came using a boat, they do not exchange it in the trade ( since a boat is useless in the middle of desert, the Saharan Traders find no use in buying it). The main commodities that being exchanged in the trade is Salt and Gold. During the process, there is an exchange in religion culture ( the european bring Christianity to Africa and the African spread muslims to the European)