The correct answer to the question "What causes a 'V' shaped valley?" is Option 2: River erosion.
A 'V' shaped valley is formed by the erosive action of a river or stream over time. As the river flows, it erodes the surrounding rocks and sediments, gradually deepening and widening its channel. The erosive forces of the water, combined with the movement of sediment and debris carried by the river, carve out a distinct 'V' shape.
The process of river erosion involves the wearing away of the riverbed and banks through several mechanisms. Firstly, hydraulic action occurs when the force of the moving water dislodges and carries away loose particles of rock and soil. Secondly, abrasion or corrasion happens as the river's load of sediments scours and rubs against the riverbed, further deepening and widening the channel. Finally, corrosion or solution occurs when the river's water chemically dissolves certain types of rocks, such as limestone.
Over time, these erosive processes shape the landscape, creating a 'V' shaped valley with steep sides and a narrow bottom. The steep sides of the valley are known as valley walls or valley slopes, while the narrow bottom is called the valley floor or channel.
It is important to note that while glacial erosion can also create U-shaped valleys, wind erosion and mass wasting (such as landslides or rockfalls) are not typically responsible for the formation of 'V' shaped valleys.