The correct answer is: a) Coral reefs.
The Gulf of Mannar Marine National Park is a marine park located in the Gulf of Mannar, off the eastern coast of India. It is the first marine park in India and the second in Asia. The park is known for its rich coral reefs, which are home to a variety of marine life, including fish, turtles, and dolphins. The park also has a number of mangrove forests, which are important for protecting the coastline from erosion.
Option b) Mangroves is incorrect because the Gulf of Mannar Marine National Park is not known for its mangroves. Mangroves are found in other parts of India, such as the Sundarbans National Park.
Option c) Beaches is incorrect because the Gulf of Mannar Marine National Park is not known for its beaches. There are many beautiful beaches in India, such as the beaches of Goa.
Option d) Hilly landscapes is incorrect because the Gulf of Mannar Marine National Park is not known for its hilly landscapes. The park is located on a flat plain.