Introduction:
The Wildlife Protection Act of 1972 is an important legislation in India that aims to protect and conserve wildlife and their habitats. It classifies animals into different schedules based on their level of protection. Schedule 4 of the Wildlife Protection Act includes the most vulnerable and critically endangered species that require strict protection and conservation efforts.

Explanation:

Gharial:
The correct answer to the question is option 'D' - Gharial. The Gharial (Gavialis gangeticus) is a large crocodilian species native to the Indian subcontinent. It is known for its long, slender snout, which is specialized for catching fish. Gharials are primarily found in the rivers of India and Nepal, and their population has drastically declined over the years due to habitat loss, pollution, and illegal hunting. As a result, they are listed under Schedule 1 of the Wildlife Protection Act, which provides them with the highest level of protection.

Other options:

a) Indian elephant: The Indian elephant (Elephas maximus indicus) is listed under Schedule 1 of the Wildlife Protection Act. It is a highly revered and iconic species in India, and efforts are being made to protect its population and habitat.

b) Indian rhinoceros: The Indian rhinoceros (Rhinoceros unicornis) is also listed under Schedule 1 of the Wildlife Protection Act. It is one of the largest rhinoceros species and is found in the northeastern regions of India and Nepal.

c) Vultures: Vultures are not listed under Schedule 4 of the Wildlife Protection Act. However, they are listed under Schedule 1, as some vulture species in India have faced a significant decline in population due to the use of veterinary drugs in cattle, which has led to their poisoning.

Conclusion:
In conclusion, the animal listed under Schedule 4 of the Wildlife Protection Act, 1972 is the Gharial. This classification highlights the vulnerable status of the species and the need for strict protection and conservation efforts to ensure their survival in the wild.