The correct answer is option 4, Gully erosion. Gully erosion refers to the process of water cutting deep channels or ravines into the soil. It occurs when water flow concentrates in narrow paths and gains enough energy to erode and transport soil particles. In the case of Chambal Ravines, they are formed due to the continuous process of gully erosion by the Chambal river. Over time, the river has carved out deep channels and ravines in the land, resulting in the unique landscape of the Chambal Ravines. It is important to note that the other options, splash, sheet, and rill erosion, are also forms of soil erosion, but they are not responsible for the specific formation of the Chambal Ravines.