What is the main role of a water divide?

Correct answer: Separating two river basins

The direct answer is option A, separating two river basins. A water divide is a highland, ridge, or elevated area that separates the flow of rainwater into two different drainage basins. A drainage basin is the area from which a river and its tributaries collect water. For example, water falling on one side of a ridge may flow into one river system, while water falling on the other side flows into another system. Thus the main work of a water divide is to mark the boundary between two drainage areas. Option A is correct because it describes this exact function. Option B, joining a river to the sea, is not the work of a divide; a river channel carries water towards a lake or sea. Option C, forming distributaries in a delta, happens near a river mouth when the main channel divides, not on a water divide. Option D, making a lake salty, is unrelated to drainage boundaries and depends on evaporation, minerals, and water balance. Remember that a divide divides water flow, not joins it. Exam cue: high ridge plus two directions of runoff means water divide.