Final answer:
To warm up when cold, the arrector pili muscles contract, causing hairs to stand upright and form goosebumps, aiding in heat conservation. The sweat glands do not produce sweat in this scenario, and other processes like vitamin D synthesis or melanin production have different purposes.
Step-by-step explanation:
In response to the student's question regarding what happens in the body to warm it up when cold, the correct answer is D) the arrector pili muscles contract to stand hairs upright. This action is the body's response to conserve heat. When the arrector pili muscles contract, they move hair follicles and lift hair shafts, trapping more air under the hairs to provide insulation on the skin's surface. The formation of goosebumps is due to these contractions of the arrector pili muscles.
It's important to know that when the body is too cold, sweat glands do not produce sweat since their role is to cool the body when it's overheated by releasing sweat, which evaporates and cools the skin. Additionally, the constriction of blood vessels in the skin (vasoconstriction) helps to conserve heat. On the other hand, vitamin D synthesis, sweat and oil (sebum) production, and melanin production are processes that serve different functions unrelated to the immediate warming of the body.