Which of the following is a classic symptom of heat exhaustion?

Rapid heart rate is a classic symptom of heat exhaustion because, as the body becomes overheated, it works harder to cool itself down through increased circulation. This acceleration in heart rate occurs in response to the higher body temperature and the effort to redistribute blood to the skin, enabling heat loss through sweat. Other symptoms that often accompany heat exhaustion include excessive sweating, weakness, dizziness, and nausea.

The other symptoms listed, such as severe headache, dry mouth, and chest pain, can be associated with various medical conditions, but they are not the hallmark indicators of heat exhaustion specifically. Severe headache may suggest dehydration or other types of heat-related illnesses like heat stroke. Dry mouth can occur in severe dehydration but isn't a defining feature of heat exhaustion itself. Chest pain, while serious, is more indicative of cardiac issues and does not directly relate to the physiological responses of heat exhaustion.