The correct answer is option-3: Rock dam. A rock dam is a method of soil conservation where rocks are piled up to slow down the flow of water. This is done to prevent gullies and further soil loss. When water flows downhill, it can cause erosion and wash away fertile topsoil. By creating a rock dam, the flow of water is interrupted and the sediment carried by the water settles behind the dam. This helps in retaining the soil and preventing its loss. Mulching, option-1, is a different method where a layer of organic material is placed on the soil surface to protect it and conserve moisture. Contour barriers, option-2, are barriers constructed along the contour lines of the land to slow down water runoff. Terrace farming, option-4, is a technique where sloping land is transformed into a series of flat terraces with steps to prevent soil erosion.