Which of the following animals is not viviparous?

Platypus is not viviparous. Duck-billed platypus (Ornithorhynchus), is a semiaquatic, egg-laying (oviparous) mammal endemic to eastern Australia, including Tasmania. It is a connecting link between reptiles and mammals. It shows mammalian characters like mammary glands, hair, diaphragm, single aortic arch etc. It shows reptilian characters in having a large coracoid in pectoral girdle and in laying eggs with yolk and shell.

Whale, bat and elephant are viviparous (give birth to live young ones).