Final answer:
The skin's main functions include preventing water loss, serving as a barrier, synthesizing vitamin D, and regulating body temperature. Fat storage is not a function of the skin.
Step-by-step explanation:
The skin's main functions include preventing water loss from the body, serving as a barrier to the entry of microorganisms, synthesizing vitamin D, blocking UV light, and helping to regulate body temperature. One of the given options, A. fat storage, is not a function of the skin. The primary function of the skin is not storing fat, but rather protecting the body and regulating various physiological processes.