The right option is; B. Has a low stream gradient
Has a low stream gradient is the characteristic that describes an old river.
A river is a large, natural and often winding stream which carry water down from higher areas to a lower point, ending at an ocean or in an inland sea. Some rivers are young while some are old. Rivers possess different features as they age. Old rivers usually have a low stream (gentle) gradient and low erosive energy. They usually have broad U shape, the water is usually brown and they have flood plains. Seasonal floods are also common.