Explanation:

The Ganges river dolphin (Platanista gangetica gangetica) is a unique freshwater dolphin species native to the Indian subcontinent. Let's analyze each statement: \nOption A, 'They are found only in fresh water', is true. Ganges river dolphins are obligate freshwater species, meaning they exclusively inhabit freshwater river systems, including the Ganges, Brahmaputra, Meghna, and Karnaphuli-Sangu rivers and their tributaries. \nOption B, 'They are blind', is true. These dolphins have rudimentary eyes that lack a lens, rendering them functionally blind. They navigate and hunt using echolocation, emitting ultrasonic sounds and interpreting the echoes. \nOption C, 'They are in the category of endangered species', is true. The International Union for Conservation of Nature (IUCN) Red List classifies the Ganges river dolphin as 'Endangered' due to significant population declines caused by habitat degradation, pollution, dam construction, and accidental entanglement in fishing nets. \nOption D, 'They are found in abundance in Ganga river', is not true. This statement is false. Due to the threats mentioned above, their populations have drastically declined, and they are now considered rare and fragmented, certainly not found in abundance. Their numbers are critically low, making their conservation a high priority. Therefore, this is the statement that is 'not true' about Ganges river dolphins.