Answer:
Heat
Explanation:
A stimulus is anything that induces a specific reaction in a living system. Stimulus causes living organisms to respond because they are effecting a change in the physiological functioning of that organism's cell. However, a stimulus can be internal or external depending on where it is coming from.
An external source of stimuli is that which is coming from outside the body of the organism. In the question given, heat is the appropriate answer because heat is the only option that can be generated externally. Hunger is an internal stimuli caused when the organism is low in energy. Thirst is also an internal stimuli caused by lack of water in the body system. Pain is an internal response of an organism to certain conditions that are unfavorable.
