Which of the following is a key sign of heat exhaustion?

In the context of heat exhaustion, cold and clammy skin is indeed a key sign. Heat exhaustion typically occurs when the body is unable to regulate its temperature effectively due to excessive heat exposure and loss of fluids, primarily through sweating. The skin may feel damp and cool because the body is trying to cool itself down. This is indicative of the body's response to high temperatures, where blood vessels dilate and blood is directed to the skin to aid in cooling through evaporation.

Hot and dry skin is more commonly associated with heat stroke, a more severe condition where the body fails to regulate its temperature. Vomiting blood is not a symptom of heat exhaustion and would signal a different medical emergency that requires immediate attention. A strong, rapid pulse can be present in both heat exhaustion and heat stroke but is not specific to heat exhaustion alone. The presence of cold and clammy skin is a distinctive feature of heat exhaustion, aiding in its identification and the implementation of appropriate interventions.