Final answer:
The integumentary system includes the skin, hair, and nails, all of which contribute to protection and maintaining homeostasis. The trachea is not part of this system but is instead part of the respiratory system.
Step-by-step explanation:
The integumentary system includes various components that play a vital role in protecting the body and maintaining homeostasis. This system consists primarily of the skin, which is the body's largest organ system, and its accessory structures – the hair and nails. These structures are composed primarily of keratin, a tough protein that adds strength and resilience. Their primary function within the integumentary system is to provide protective covering for the body. Of the options given, hair (a), fingernails and toenails (b), and skin (c) are certainly part of the integumentary system. However, the trachea, option (d), is not a component of the integumentary system but rather part of the respiratory system, playing a major role in air passage.