Which one of the following is not a mammal ?

Among the given options, the shark is the only animal that is not a mammal. Sharks are cartilaginous fish that obtain oxygen through gills rather than lungs [t4]. Unlike mammals, sharks lack mammary glands and do not nurse their young with milk [t1]. They are cold-blooded and possess scales, whereas mammals are warm-blooded and typically have hair or fur [t4]. In contrast, dolphins and whales are cetaceans, which are aquatic mammals that breathe air through blowholes and nurse their young [c1, t7]. Seals are pinnipeds, another group of marine mammals that share core mammalian traits such as being warm-blooded and giving birth to live young [t4]. While some sharks give birth to live young, they do not provide parental care or nourishment via milk, which are defining characteristics of the class Mammalia [t1, t8].

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Do you know? > p. 154