The correct answer is option 2: Gulf of Mannar biosphere reserve. This biosphere reserve is located in Tamil Nadu, India and comprises islands with estuaries, beaches, coral reefs, salt marshes, and mangroves.

The Sunderban biosphere reserve (option 1) is located in West Bengal, India and is known for its mangrove forests, but it does not include the other mentioned habitats like estuaries, beaches, and coral reefs.

The Nilgiri biosphere reserve (option 3) is located in the Western Ghats of India and includes several protected areas. While it contains diverse habitats like forests and grasslands, it does not include islands with estuaries, beaches, coral reefs, salt marshes, and mangroves.

The Nandadevi biosphere reserve (option 4) is located in the state of Uttarakhand, India and is known for its high-altitude peaks and glaciers. It does not include islands with estuaries, beaches, coral reefs, salt marshes, and mangroves.

Therefore, the Gulf of Mannar biosphere reserve is the correct answer as it includes all the mentioned habitats and is specifically known for its rich marine biodiversity.