Final answer:
Beetles use only one pair of their two pairs of wings for flight—the second pair known as elytra—while their first pair of wings acts as protective covers.
Step-by-step explanation:
The insect that uses only one pair of wings for flying is d) beetles. Beetles have two pairs of wings, but they use only the second pair, known as elytra, for flight. The first pair of wings is hardened and serves as a protective cover for the delicate flying wings and the beetle's abdomen. While other insects such as butterflies, dragonflies, and moths also have wings, they use both pairs for flying.
It's important to note that form and function in insect wings can vary greatly. For example, butterflies and katydids use their wings not only for flight but for other functions such as camouflage, communication, and mate attraction. In contrast, the beetle's hardened wing covers provide protection when they are not flying.