Option (A) Youth stage is correct because the textbook clearly states that 'In the early stages, down-cutting dominates during which irregularities such as waterfalls and cascades will be removed.' The Youth stage corresponds to the early stage of landform development where vertical erosion or downward cutting is the dominant process. In later stages (middle/mature stages), the streams cut their beds slower and lateral erosion of valley sides becomes severe, meaning downward cutting is no longer dominant.