Final answer:
Respiration is not a primary function of the skin; the primary functions include protection, sensation, thermoregulation, absorption, secretion, and vitamin D synthesis. Therefore, the correct option is C.
Step-by-step explanation:
The primary functions of the skin include protection, sensation, thermoregulation, absorption, secretion, and synthesis of vitamin D. Among the options given, respiration is not considered a primary function of the skin. While the skin does allow a minimal amount of gas exchange, it is not sufficiently significant to be categorized as a primary function like the gas exchange that occurs in the lungs. Therefore, respiration does not directly contribute to the protective function of the skin.