Final answer:
When body temperatures rise, dermal blood vessels dilate to allow heat to radiate from the skin, preventing the body from overheating. The correct option is a.
Step-by-step explanation:
When body temperature rises above the normal set point, the correct response is: a. Dermal blood vessels dilate. This process, known as vasodilation, allows more warm blood to flow close to the surface of the skin, enabling heat to be radiated into the environment. The dilation of dermal blood vessels is part of the body's thermoregulatory response to prevent overheating. This physiological change helps maintain homeostasis by expelling excess body heat through the skin.