Explanation:

- Damodar Valley: This region holds a significant portion of India's coal reserves. It's commonly referred to as India’s Ruhr as it was one of the earliest coal mining areas.

- Son Valley: It's known for its fertile lands and minerals but isn't as prominent in coal reserves compared to others.

- Mahanadi Valley: This region has coal reserves but doesn't match the prominence of the Damodar Valley.

- Godavari Valley: Also known for coal reserves but again, not as extensive as Damodar.

Correct Answer: Damodar Valley