C. what are the characteristics of an old river?

Explanation:

An old river occurs at the plain course and it is at its lower stage.

Here:

The river gradient is very gentle as it is on a plain. An old river would be at its depositional phase where all the sediments its carries are being laid down. It has a low velocity and a very sluggish flow. Lateral erosion is very active at this stage. The volume of the river increases substantially as more river tributaries joins. Some of the river features are flood plains, leeves, ox-bow lakes, delta e.tc.