Final answer:
A river in a stable climate may reach a dynamic equilibrium and is known as a graded stream, which maintains a balance between its carrying capacity and sediment load without significant erosion or deposition. The correct option is B.
Step-by-step explanation:
When conditions such as climate remain stable, a river may approach a dynamic equilibrium state. The river would then be considered a graded stream.
A graded stream is a channel that has adjusted its slope, shape, and the balance between its carrying capacity and the sediment load it carries to reach a state of equilibrium.
This means that the energy available due to the stream's gradient is just enough to transport the sediment load provided by its drainage basin without either eroding or depositing material, maintaining a delicate balance.
A progressive stream is not a commonly used term in this context, and a tributary is a smaller stream or river that flows into a larger one, which does not specifically describe the equilibrium status of the river itself.
Graded streams behave in a way that if disrupted, they tend to return to a state of equilibrium, showcasing the ecosystem's ability to remain in equilibrium despite disturbances like a beaver damming a river upstream or a forest fire. The correct option is B.