The correct answer is C) Heat stroke. In heat stroke, the individual has lost the ability to lose heat through the mechanism of sweating. This is because heat stroke is a medical emergency where the body's temperature regulation system fails, causing the body temperature to rise rapidly. During heat stroke, the sweat glands stop functioning, leading to a loss of the body's main cooling mechanism. This distinguishes heat stroke from other heat-related illnesses like heat cramps (A) and heat exhaustion (B), where sweating is still present as the body attempts to cool itself.