Answer: B. Its floodplain widens
As, the river gets older it accumulates larger number of water sources called as tributaries. Therefore, the volume of water will increase. Also, the soil near the water body is taken away by water flowing in a faster current due to soil erosion by water. This soil gets settled at the bottom increasing the channel of river flow. As, the river gets older the bottom bed becomes wider hence, will result in increased in water channel. Therefore, floodplains are wider in rivers getting older.