Option (D) Erosion is the correct answer. According to the textbook context, 'The phenomenon of wearing down of relief variations of the surface of the earth through erosion is known as gradation.' This directly establishes erosion as a gradational process. The other options are incorrect because: Diastrophism and Volcanism are explicitly stated as 'endogenic geomorphic processes' (land building forces), not gradational processes. Deposition, while an exogenic process, is associated with aggradation (filling up of basins) rather than gradation (wearing down of relief). Gradation specifically refers to the wearing down of elevations through erosion.