Explanation

A mature (old-stage) river flows with a very gentle gradient across a broad floodplain, depositing sediment and developing pronounced meanders and oxbow lakes. Young rivers are steep, fast and erosional (V-shaped valleys, waterfalls), while old rivers are sluggish and depositional. Option B correctly describes this old-stage character, whereas A and C describe youthful mountain streams and D describes a middle/old-stage stream with pools and riffles.