During the youthful stage of a river, the process that takes place is valley deepening. This means that the river is actively eroding and cutting into the land, deepening the valley in which it flows. This process is typically characterized by a steep profile, fast-moving water, and the formation of V-shaped valleys. Valley widening, on the other hand, occurs during the mature or old age stage of a river. River rejuvenation refers to the process of a river becoming re-energized by tectonic uplift, but it is not specific to the youthful stage. Meandering typically occurs in the mature stage of a river, where the river starts to develop bends and curves. Therefore, the correct answer is option 3, valley deepening.