Widening of a river valley occurs due to lateral erosion, which is the correct answer. Lateral erosion refers to the sideways erosion of the river banks. As the river flows, it exerts a force against the sides of the river valley, leading to the removal and transportation of sediment. Over time, this continuous erosion widens the river valley.

Option 1, corrosion, refers to the chemical erosion of rocks, which is not the primary cause of river valley widening.

Option 3, corrasion, is not a widely recognized term in geology, so it is unlikely to be the correct answer.

Option 4, hydraulic action, refers to the mechanical erosion caused by the force of flowing water, which can lead to the widening of a river valley. However, lateral erosion is a more specific and accurate term for this process.

Therefore, the correct answer remains option 2 - lateral erosion.