"why is the climate drier on the leeward (downwind) side of mountain ranges that are subjected to prevailing winds? "

The leeward side of the mountain is drier because the rainfall generally stops on the other side of the mountain, so the leeward side gets much less or no rain at all because the clouds are stopped by the mountain. So instead of rain, they get winds, dry winds, leftovers of the air mass that hit the other side of the mountain, that blow downhill and make the soil dry.

How is this even a question?

Well, a and b together would be negative. b alone would also be negative.