Which of the following is a sign of hypothermia?

Hypothermia occurs when the body loses heat faster than it can produce it, causing a dangerous drop in body temperature. One of the primary indicators of hypothermia is uncontrollable shivering, which is the body's attempt to generate heat. Additionally, pale or cold skin is another common sign as the blood vessels constrict in response to the cold to preserve core body temperature.

In contrast, the other choices include symptoms that are not associated with hypothermia. Increased heart rate and warm skin suggest that the body is functioning normally and is effectively warming itself, which is not characteristic of hypothermia. Normal skin temperature and clarity of mind indicate that the individual is not experiencing the effects of hypothermia. Rapid breathing and headache can occur due to various reasons but are not definitive signs of hypothermia, making the combination of uncontrollable shivering and pale skin the most accurate indicator for this condition.