Detailed Explanation:

✅ Statement 1 – Correct: Andaman and Nicobar Islands have extensive fringing coral reefs along their coasts, forming one of India's richest marine biodiversity zones.

✅ Statement 2 – Correct: Gulf of Kutch contains coral reefs in the northern Arabian Sea, declared as India's first Marine National Park in 1982.

✅ Statement 3 – Correct: Gulf of Mannar between India and Sri Lanka hosts 21 islands with fringing coral reefs, recognized as a Marine Biosphere Reserve.

❌ Statement 4 – Incorrect: Sundarbans is a mangrove delta ecosystem with brackish water and muddy substrate, unsuitable for coral reef formation which requires clear saline water.