The correct answer is c) Ganges river dolphins.
The Vikramshila Gangetic Dolphin Sanctuary is a protected area in India that is dedicated to the conservation of the Ganges river dolphin. The sanctuary is located in the Ganges River, which flows through India and Bangladesh. The Ganges river dolphin is a freshwater dolphin that is found in the Ganges River and its tributaries. It is the only freshwater dolphin species found in Asia. The Ganges river dolphin is a threatened species, and the sanctuary is an important effort to protect it.
The other options are incorrect because they are not freshwater dolphins. Irrawaddy dolphins are found in coastal waters in Southeast Asia. Bottlenose dolphins are found in coastal waters around the world. Spinner dolphins are found in tropical and subtropical oceans around the world.