The marine animal called dugong which is vulnerable to extinction is:

Answer: (d) Mammal

The dugong breathes air, gives birth to live young and feeds them milk — a marine mammal (order Sirenia).

The idea

An animal that lives in water is not necessarily a fish. The test for a mammal: it breathes air with lungs, gives birth to live young and feeds them on milk. The dugong does all three and must come to the surface to breathe.

Whales and dolphins → Dugong and manatees (sea cows) → Seals, sea lions and walrus → Sea otter and polar bear

Also know

These are the groups of marine mammals. The dugong and the manatees form one small order; sailors’ stories of mermaids are thought to have begun with them.