I believe the most VALID answer to your question is "Homeostasis".
Organisms that carry out homeostasis do so in order to maintain a constant, optimum environment for the cells within their body.
It would be C because hair and feathers and what not are made from the protein keratin.
<span>vasoactive intestinal peptide</span>
<span>The answer is smooth endoplasmatic reticulum. Smooth endoplasmic reticulum is involved in the production and metabolism of lipids and in the production of steroid hormones. Also, smooth endoplasmic reticulum converts organic chemicals to water-soluble products and that way detoxifies them.</span>