What is the primary function of the skin?

The primary function of the skin is regulating temperature. The skin plays a critical role in thermoregulation through processes such as sweating and vasodilation or vasoconstriction of blood vessels. When the body temperature rises, sweat glands produce sweat, which evaporates from the skin's surface, cooling the body down. Conversely, when the body is cold, blood vessels constrict to minimize heat loss.

While protecting internal organs is an important role of the skin, it serves primarily as a barrier and protective layer against environmental hazards rather than its main function. The production of hormones, such as vitamin D synthesis, is another function of the skin, but it is secondary to temperature regulation. Generating energy is not a recognized function of the skin, as energy production primarily occurs at the cellular level in the body's metabolic processes. Therefore, temperature regulation stands out as the most significant function among the choices given.