In river erosion, the youthful stage is characterized by the formation of a gorge. A gorge is a deep and narrow valley with steep sides, usually formed by the erosion of the river cutting through the land. During this stage, the river has a high velocity and a strong erosive power, as it is at a higher gradient. This allows the river to cut deeply into the land, creating a gorge.

Options 1, 3, and 4 are not typical features of river erosion in the youthful stage. An ox-bowlake is formed by the deposition of sediment in a meander bend of a river, not by erosion. A valley in a valley is not a characteristic feature of the youthful stage, as the river is still in the process of deepening its main valley. A cut-bank is formed by the lateral erosion of a river, with the outer bank being eroded, not a typical feature of the youthful stage.