Which one of the following is the national aquatic animal of India?

The correct option is Gangetic dolphin.

Let's understand why this is the correct answer step-by-step:

1. Official Declaration: The Ganges River Dolphin (scientifically known as Platanista gangetica) was officially declared as the National Aquatic Animal of India in the year 2009 by the Government of India. This decision was taken during the first meeting of the National Ganga River Basin Authority (NGRBA) to save the species from extinction and to serve as an indicator of the health of the Ganges River ecosystem.

2. Characteristics of the Gangetic Dolphin:
- It is a freshwater dolphin species that primarily inhabits the Ganges, Yamuna, Chambal, Brahmaputra, and Meghna rivers in India, Nepal, and Bangladesh.
- They are functionally blind and hunt using echolocation (sending out sound waves that bounce off objects, allowing them to create a mental image of their surroundings).
- They are also locally known as 'Susu' due to the noise they make while breathing when they surface.

3. Evaluating the Other Options:
- Saltwater crocodile: While found in India (primarily in Odisha's Bhitarkanika and the Sundarbans), it is not designated as the national aquatic animal.
- Olive ridley turtle: These marine turtles nest in massive numbers on the Odisha coast, but they are not the national aquatic animal.
- Gharial: A critically endangered freshwater crocodilian native to the Indian subcontinent, it also does not hold the national aquatic animal status.

Therefore, the Gangetic dolphin is officially designated as the national aquatic animal of India, representing the purity of the holy river Ganga as it can only survive in pure and fresh water.