The correct answer is option 1: Lateral Corrosion. Lateral corrosion refers to the sideways erosion that occurs along the banks of a river, gradually widening the river valley over time. As the river flows, its water currents exert pressure against the sides of the valley, wearing away the rock and soil. This constant sideways erosion causes the river valley to widen as the banks are eroded and the valley floor is lowered. The process of lateral corrosion is a natural and ongoing phenomenon in river systems and is one of the main factors that shape the landscape. It is important to note that the other options, vertical corrosion, side corrosion, and mean corrosion, do not accurately describe the specific type of erosion that widens river valleys.