In which stage does an insect undergo major changes from larval to adult form?

The stage in which an insect undergoes major changes from the larval to adult form is the pupa stage. During this phase, the larva transforms within a protective casing and undergoes significant biological changes, including the reorganization of its body structure. This metamorphosis results in the development of adult characteristics, allowing the insect to emerge in its fully formed adult stage.

In contrast, options like nymph and larva represent other life stages. The nymph stage is typically found in insects that undergo incomplete metamorphosis, where they resemble small adults rather than going through a pupal stage. The larval stage is primarily focused on growth and feeding rather than transformation. The egg stage represents the earliest life cycle development before any significant changes have occurred. Thus, the pupa stage is critical for the complete metamorphosis of insects, making it the correct choice for indicating major changes to adulthood.