Final answer:
All three offered options—avian feathers, reptilian scales, and mammalian hair—are structures made of keratin, which is a key structural protein that provides strength and protection to various parts of the vertebrates' bodies.option e is correct.
Step-by-step explanation:
Structures that are made of keratin include avian feathers, reptilian scales, and mammalian hair. Keratin is a tough, fibrous protein that is a key structural component of the outer layer of the skin of vertebrates. It provides strength, protection, and can help in minimizing water loss.
Avian feathers are essential for flight and thermal insulation in birds, while reptilian scales help in protection and prevention of water loss. Mammalian hair, which is also made up of keratin, serves several functions such as providing insulation, serving as a filter, and aiding in camouflage and mate attraction.