What characteristic describes the term "exuvium"?

The term "exuvium" refers specifically to the cast skin that is left behind after an insect undergoes molting. This process is essential for insects as it allows them to grow and develop from one life stage to another. During molting, the insect sheds its outer layer to allow for the expansion of its body, and the exuvium is the remnant of this old exoskeleton. Understanding this term is crucial for recognizing the physical changes insects go through at various points in their life cycles, and it highlights the fascinating adaptations of arthropods in managing their growth.

The other choices do not accurately capture the meaning of "exuvium"—habitat refers to where an organism lives, a reproductive phase pertains to the stages related to reproduction, and age indicates how long an insect has been alive, none of which are relevant to the concept of an exuvium as a physical cast skin.