The correct answer is - D. Deposition.
Sand dunes are formed by depositions of send grains in the desertous parts of the world. The strong winds in the deserts slowly contribute to the weathering of the rocks, the small particles of sand that are formed from it are than taken by winds and transported, once the winds start to lose their speed they do not have the required force to keep hold of the sand grains and they fall on the ground, thus deposit them. Over time, multiple layers of deposited sand grains form hills of sand that are called sand dunes.