Which type of river is characterized by a gentle slope and carries smaller particles due to lower energy?

The choice of a mature river as the correct answer is based on its characteristics relative to the others in the list. A mature river features a gentle slope, which means its gradient is less steep compared to younger rivers. This gentle slope results in a lower energy flow of water, which is unable to carry larger particles and therefore transports smaller sediment such as silt and clay.

In contrast, a youth river, with its steeper gradient, typically has a faster flow that allows it to carry larger rocks and debris. An old age river may have evolved into a more complex system with features like floodplains and swamps, but it does not retain the characteristics of a gentle slope and smaller particle transportation as effectively as a mature river. Meandering rivers, while they can also indicate past maturity, can exhibit varied gradients and energy levels; they are known for their winding paths rather than just the sediment they carry.

Thus, a mature river is distinctly characterized by its gentle slope and capacity to transport smaller particles due to lower energy flow.