The correct answer is option 3: wind deposition. Wind deposition is the process responsible for producing the sand dunes in western Rajasthan. This process occurs when wind carries and deposits sand particles in a particular area. As the wind blows, it picks up loose particles of sand from the ground, especially in arid and desert regions like western Rajasthan where there is limited vegetation to hold the sand in place. When the wind loses its energy, it drops or deposits the sand particles, creating sand dunes. These sand dunes can vary in size, shape, and orientation depending on the strength and direction of the wind. Erosion by water (option 2) is not the primary process responsible for sand dune formation in this region.