Final answer:
The thermoneutral zone for adults is typically around 20-30°C, which corresponds to a range where the human body does not need to expend energy to maintain its setpoint core temperature of approximately 36.5-37.5°C.
Step-by-step explanation:
The thermoneutral zone for adults is a range of external temperature in which the body does not expend any energy for thermoregulation. This is because the surrounding temperature is adequate to maintain the body temperature setpoint without the need for extra physiological adjustments.
The thermoneutral zone varies among species, but for human adults, the body maintains its core temperature steadily at around 36.5-37.5 ℃ (or 97.7-99.5 ℉). Given the provided options and considering the thermoneutral external temperature at which the body does not expend any energy for thermoregulation is about 84 ℉ (approximately 28-29 ℃), the correct answer to the question is a. 20-30℃.