The correct answer is: a) Temple tank.
Theerthavari is a Hindu ritual of taking a holy dip in a sacred water body. It is believed that this ritual cleanses the soul and brings good luck. Theerthavari is often performed at temples, where there are usually temple tanks or wells that are used for this purpose.
The temple tank at Rameswaram is one of the most sacred water bodies in India. It is believed that the water in this tank has healing properties. Theerthavari at the Rameswaram temple is a popular pilgrimage destination, and thousands of people visit the temple every year to take a holy dip in the tank.
The other options are incorrect because they are not sacred water bodies. River Kaveri is a river in India, Bay of Bengal is an ocean, and waterfalls are natural water bodies that are not considered to be sacred.