The correct answer is option 4: Beas.

The Indus Dolphin, also known as the Bhulan, is indeed one of the world`s rarest mammals. However, it is not predominantly found in the Ganga (option 1), Cauvery (option 2), or Brahmaputra (option 3) rivers.

The Indus Dolphin is primarily found in the Indus River, which flows through Pakistan. However, small populations of the Indus Dolphin are also found in the Beas River, which is a tributary of the Indus River. The Beas River is located in the northern part of India, primarily in the state of Himachal Pradesh.

It is important to note that the Indus Dolphin population is highly endangered and faces various threats, such as habitat degradation, pollution, and fishing practices. Conservation efforts are being undertaken to protect the species and its habitat.

Alert - correct answer should be option 1: Ganga.