The correct answer is option 2: Gahirmatha. Gahirmatha, located in the Indian state of Odisha, is well known for the annual mass nesting of Olive Ridley turtles.

Option 1: Gopalpur is not known for the annual mass nesting of Olive Ridley turtles.

Option 3: Paradeep is also not known for the annual mass nesting of Olive Ridley turtles.

Option 4: Puri, although a popular tourist destination in Odisha, is not specifically known for the annual mass nesting of Olive Ridley turtles.

It is important to note that Olive Ridley turtles are known for their unique behavior of synchronized nesting, where thousands of females come ashore to lay eggs at the same time. Gahirmatha in Odisha has been identified as the largest mass nesting site for these turtles, making it a well-known and important location for conservation efforts.