The correct answer is D. Western Ghats.
The Western Ghats are a mountain range that runs along the western coast of India. They are the main watershed of most of the peninsular rivers in India. The Western Ghats are a biodiversity hotspot and are home to a wide variety of plants and animals. They are also an important source of water for the people of India.
The Aravalli hills are a mountain range that runs through the states of Rajasthan, Gujarat, and Haryana in India. They are the oldest mountain range in India and are not as important a watershed as the Western Ghats.
The Raj Mahal hills are a mountain range that runs through the states of Jharkhand and Bihar in India. They are not as important a watershed as the Western Ghats.
The Siwalik hills are a mountain range that runs along the foothills of the Himalayas in India. They are not as important a watershed as the Western Ghats.