Metamorphosis is a biological process that involves

Metamorphosis means a clear change in the body form and structure of an animal as it passes through the stages of its life cycle. The young animal does not simply get bigger — it looks and lives very differently from the adult.

In complete metamorphosis, an insect like the butterfly or housefly goes egg → larva → pupa → adult. In incomplete metamorphosis, insects like the cockroach and grasshopper go egg → nymph → adult, with the nymph looking like a small wingless adult. A tadpole turning into a frog is another good example.

The other options describe different things: growth from a zygote to an adult happens in all animals, even those without metamorphosis; regrowing a lost body part is regeneration; and the change always runs from young stage toward adult, never from adult back to a larva.