What is the correct role of a water divide?

Correct answer: It separates two drainage basins

The direct answer is B: a water divide separates two drainage basins. A drainage basin is the area from which all water flows into one main river and its tributaries. A water divide is a higher ridge, hill, or elevated boundary; rainfall falling on its two sides flows into different river systems. Thus it acts like a boundary between neighbouring basins. Option B is correct because separation of drainage basins is the defining role of a water divide. Option A is wrong because a delta forms where a river deposits sediment near its mouth; a divide is usually found in higher land near the edges of basins. Option C is wrong because a divide does not keep a river away from the sea; rivers generally flow downhill and may ultimately reach the sea. Option D is wrong because pollution is caused by harmful substances and human activities, not by the natural boundary called a divide. Think of a roof ridge: water on one side runs one way and water on the other side runs another way. In an exam, connect water divide with basin boundary.