In a state of rest, the most important humoral factor regulating heat production in the human body is
Correct answer: A. thyroxine
Thyroxine, produced by the thyroid gland, plays a crucial role in regulating basal metabolic rate and heat production during rest. Adrenaline, noradrenaline, and acetylcholine are primarily involved in stress responses and other physiological functions but are not the most important humoral factors for heat production at rest.