Which term describes immature insects that develop with functional sex organs and adult structures?

The term that describes immature insects developing with functional sex organs and adult structures is "nymph." Nymphs are a specific life stage of hemimetabolous insects, such as grasshoppers and cockroaches. In this stage, nymphs resemble smaller versions of adult insects, though they often lack fully developed wings. Unlike larvae, which are associated with holometabolous insects and undergo a significant transformation during pupation, nymphs gradually mature into adults through a series of molts, acquiring adult characteristics over time.

This distinction highlights why "nymph" is the correct choice in this context, as it specifically refers to the immature form that is more directly comparable to the adult stage than other forms like larvae, which are distinctly different. Additionally, while the terms "adult" and "pupa" refer to separate life stages, they do not describe the immature stage relevant to the question at hand.