Final answer:
The integumentary system's functions include protection, thermoregulation, and synthesis of vitamin D. It also plays a role in excretion through sweat but does not synthesize vitamin C, which must be obtained from the diet. The option (D) is correct.
Step-by-step explanation:
The integumentary system, which includes the skin, hair, and nails, performs several critical roles in maintaining the health of the body. These functions include the protection of underlying tissues from a variety of external threats such as pathogens and physical impacts, thermoregulation through sweat production and blood vessel dilation or constriction, and the synthesis of vitamin D, which aids in the absorption of calcium and phosphorus.
Excretion of waste products also occurs through sweat, though it is not the primary function of the system. The effect of sweat on cooling the body surface is a crucial part of the system's role in thermoregulation. However, the integumentary system does not synthesize vitamin C; instead, vitamin C must be obtained through dietary sources. Therefore, option (D) is correct.