Final answer:
A Rift Valley (option c) is a large, elongated depression with steep walls, formed by the downward displacement of a block of the earth's surface due to divergent tectonic forces, notably exemplified by the Great Rift Valley in East Africa.
Step-by-step explanation:
The large, elongated depression with steep walls formed by the downward displacement of a block of the earth's surface is known as a Rift Valley. A Rift Valley is created by the tensional tectonic forces which occur at divergent plate boundaries. This geological process often results in a series of unique landscapes and ecosystems, and one of the most prominent examples in the world is the Great Rift Valley in East Africa. Unlike a trench, a topographic depression usually associated with convergent plate boundaries and found on the sea floor, or a foreland basin that forms parallel to mountain belts, a Rift Valley is characterized by the splitting of the earth's crust, which creates an elongated depression.
The Great Rift Valley provides clear evidence of this, where a split in the African Plate has divided it into the smaller Somalian Plate and Nubian Plate. It's important to distinguish a Rift Valley from other geological features such as pull-apart basins, which form between two strike-slip faults, or the unique geological structures of the Basin and Range province in Nevada, which is formed through normal faulting.