Correct Option (a)
Echidna, along with the platypus, belongs to the order Monotremata, which are unique mammals known for laying eggs instead of giving birth to live young. Despite their egg-laying characteristic, monotremes possess other definitive mammalian traits such as the presence of fur and milk production (though they lack nipples, secreting milk through pores).
Incorrect Options:
Kangaroos are marsupials, a group of mammals that give birth to live, relatively undeveloped young. These young then complete their development in a maternal pouch.
Porcupines are placental mammals. They exhibit viviparity, meaning they give birth to fully developed live young after a gestation period within the mother's uterus, where the embryo is nourished via a placenta.
Whales are marine mammals and are also viviparous. They give birth to live young in water, and the calves are nursed by their mothers.