Sand is the substance carried by the wind that contributes to the weathering of rocks, as it is small enough to be transported and can erode surfaces upon impact.
Among the options provided, sand is the one that might be carried by the wind, which causes the weathering of rocks. Sand grains are the result of physical and chemical weathering and can be transported by wind, leading to abrasion and further weathering when they collide with other surfaces.
Pollen is also transported by wind but does not play a significant role in the weathering of rocks. Mud is typically too heavy to be transported by wind, and fog, while composed of water droplets, does not have the mechanical force necessary to cause weathering.
sand is the substance that both contributes to and is a product of the weathering process, as it is small enough to be carried by the wind and has the ability to cause erosion upon impact with rock surfaces.