What type of metamorphosis do flies undergo?

Flies undergo complete metamorphosis, which is a developmental process that involves four distinct stages: egg, larva (or caterpillar), pupa (or chrysalis), and adult. This type of metamorphosis is characterized by significant changes in form and function between each stage. For instance, the larval stage of a fly looks entirely different from the adult fly, as the larva is often worm-like and primarily focused on feeding, while the adult fly has wings and is capable of reproduction.

In complete metamorphosis, the pupa stage is a transformative period during which the larval body is reorganized to form the adult structure. This comprehensive transformation is a hallmark of insects like flies, making "complete metamorphosis" the correct choice. Other types of metamorphosis, such as incomplete metamorphosis, involve only three life stages (egg, nymph, adult) without a pupal stage, which does not apply to flies.