Detailed Explanation:

Kerala is India's leading producer of thorium, extracted from monazite sands found abundantly along its coastal belt, particularly in districts like Kollam and Thiruvananthapuram.

Thorium is a radioactive mineral used in nuclear fuel, ceramics, welding rods, and aerospace alloys. India holds the world's largest thorium reserves, with Kerala and Tamil Nadu accounting for the bulk.

Therefore, Kerala is the correct answer, making Option 3 correct.