Concept:River regime refers to the pattern of changes in a river’s flow over time.Explanation:A river regime describes how the volume of water in a river varies during different seasons.It is mainly determined by rainfall distribution, temperature, and drainage basin characteristics.This makes it a seasonal pattern rather than a fixed measurement of flow rate or load.Therefore, the best description is the seasonal variation in the volume of water in a river.Answer:A. seasonal variation in the volume of water in a river