The correct answer is Residual Mountain. Explanation & Key Takeaways: • Residual Mountain • Mountain which is worn down due to a process of erosion for a long time by agents like wind, air, water. Important Notes: • Graben • Graben is a depressed block of the crust of a planet due to internal disturbances. • Horst • A horst is a raised fault block bounded by normal faults. • A horst is a raised block of the Earth's crust that has lifted or has remained stationary, while the land on either side (graben) has subsided. • Rift Valley • A rift valley is a linear-shaped lowland between several highlands or mountain ranges created by the action of a geologic rift or fault. A rift valley is formed on a divergent plate boundary, a crustal extension or spreading apart of the surface, which is subsequently further deepened by the forces of erosion. Related Concepts: • Internal forces of the earth result in earthquakes and volcanoes. • The forces which originate from within the earth’s crust or inside the earth are called internal or endogenetic forces. • The sources providing them energy are the internal heat, chemical reactions taking place within the earth.