Final answer:
Downcutting streams and a youthful stage of valley evolution are characterized by V-shaped valleys.
Step-by-step explanation:
Downcutting streams and a youthful stage of valley evolution are characterized by V-shaped valleys, which are narrow and deep with steep sides. These valleys are formed as the stream erodes the land downwards. Broad floodplains are not characteristic of downcutting streams and are instead associated with mature or old age stages of valley evolution. Meandering channels are also not characteristic of downcutting streams, as they form in later stages of valley evolution. Developed deltas are typically seen in areas where a river meets a larger body of water, such as an ocean or a lake, and are formed by the deposition of sediment carried by the river. They are not characteristic of downcutting streams or youthful stages of valley evolution. Therefore, the characteristic features of downcutting streams and a youthful stage of valley evolution are V-shaped valleys.