Which of the following animals is not viviparous?
(a) Platypus
(b) Whale
(c) Flying fox (Bat)
(d) Elephant

Hint: This is one of the world’s most evolutionarily distinct mammals, one of five extant species of egg- laying mammals, and the only living species within the subclass Monotremata and the only living species within the family Ornithorhynchidae.

Complete step by step answer:
Platypus is, however, true mammals because they elaborate posh milk under the control of equivalent hormones that control lactation in both other groups of mammals. The most characteristic feature of monotremes is that they lay eggs. This is not to say that the embryo is not nourished in the uterus and they have an allantois and yolk sac placenta. The egg is laid when the embryo has reached the 19-20 somite stage, and within the echidna, but not the platypus, a pouch develops to receive the egg.

So, the correct answer is, ‘Platypus.’

Additional information:
- As one among the world’s most evolutionarily distinct mammals, the platypus has long been regarded to be of outstanding scientific importance also as a globally unique component of Australia’s biodiversity.
- The young are dependent on milk for the first few months of life to sustain its development.
- In platypus, like all other mammals, the milk is produced by well- developed mammary glands.
- Unlike all other mammals, the feminine s haven't any nipples and therefore the milk is expressed by the female from two teatless areolae and the young suck the milk directly from the abdominal surface.
- Monotreme milk composition, is similar to that of marsupials, changes throughout lactation and therefore the composition of platypus and echidna milk differ significantly.
- Most organ systems are known to differentiate during pouch or nest life although much less is understood about this process than in marsupials.
- The platypus karyotype consists of 52 chromosomes in both sexes and includes six pairs of huge autosomes, an outsized X chromosome, 17 pairs of smaller chromosomes that are hard to differentiate and four small elements with no pair.

Note:
- Two unique features of monotremes are the development of an electro- sensory mechanism in their snouts that assists them to locate their invertebrate prey and the presence in males of a keratinous canalized spur which are connected by a skinny membranous duct to an associated venom gland.
- In the platypus, the venom system is well developed and which are capable during the breeding season of inflicting wounds, but not apparently death, during territorial fighting between males.