Final answer:
The arrector pili muscle causes the hair to stand on end (option C), leading to what is commonly known as goosebumps, and is not involved with the rate of hair growth.
Step-by-step explanation:
The arrector pili muscle is a tiny muscle found in the skin that reacts to certain stimuli, such as cold temperatures or emotional stress, by contracting. This contraction pulls on the hair follicles, making the hair that is within these follicles stand on end. This causes what we commonly refer to as goosebumps. Therefore, the correct answer to the student's question is C) the hair to stand on end. These contractions are an evolutionary response that in animals with thicker fur would increase insulation. However, in humans, who typically have less body hair, the effect is less pronounced. The arrector pili muscles are controlled by the sympathetic nervous system and are not related to the rate of hair growth.