And, no need to hope William S. You are correct. The answer is D. In short, travel by sea became much easier, less dangerous, and more profitable than travel through the vast Sahara. The logistics of travel through the desert (heat, lack of water, etc.), unrest along the Trans-Saharan routes (Ex: Moroccan attack on Songhai empire-Timbuktu), and advancements in maritime travel, along with shifting economic and political contexts all combined to the decline of Trans-Saharan trade.
I'm sticking my neck out here, but I believe the answer is d. As the Portuguese sailors became bolder and bolder they discovered (or, perhaps, uncovered) more and more of the west African coastline. North Africa became less important as West Africa increased in importance as a trade route.