Which of the following greatly impairs the body's ability to lose heat?

Being overweight significantly impacts the body's ability to lose heat due to the increased insulation provided by excess body fat. Body fat serves as an insulator, which reduces the effectiveness of heat dissipation through processes like conduction, convection, and radiation. This means that individuals with higher body fat may struggle to release heat effectively, resulting in an increased risk of heat-related illnesses, especially in hot environments.

The other choices can also influence heat regulation, but the degree to which they impair the body's cooling mechanisms varies. High humidity can hinder the evaporation of sweat, reducing cooling effectiveness, while intense exercise generates heat, requiring the body to dissipate that heat. Excessive clothing can trap heat and limit air circulation but does not necessarily have the same long-term implications for heat regulation as being overweight does. Therefore, weight can be a persistent and significant barrier to effective thermoregulation in various conditions.