Concept:A circular depression cut into the rocky bed of an upper course river by erosive action is known as a pothole.
Explanation:In the youthful or upper stage of a river, the water flows rapidly over an uneven, hard rock surface.
Small stones, pebbles, and gravel are caught in swirls or eddies of the fast-moving water.
These particles are whirled around within small hollows or cracks in the river bed, acting like grinding tools.
This continuous abrasive action, called corrosion or abrasion, scours the rock surface deeper and wider.
Over a long period, the repeated grinding and rotation of the stones carves out a smooth, circular, basin-like hollow in the river bed.
This eroded depression is called a pothole, and it is a typical landform of river erosion in the upper course where the stream has great energy.
Answer:D. Potholes