All of the statements describe responses to external stimuli. However, they can be distinguished based on the nature of the stimuli:
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A rabbit runs away from a fox that is chasing it. - This is a response to the threat posed by the fox (predatory stimulus).
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A woman drinks water when she is thirsty. - This is a response to an internal stimulus (the body's need for hydration).
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A person yawns when oxygen levels in the blood are low. - This is also a response to an internal stimulus indicating a physiological need for oxygen.
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A boy eats food when he is hungry. - This is another response to an internal stimulus (the body's need for nourishment).
If you're looking for a pure response to external stimuli, the first statement about the rabbit running from the fox is the clearest example as it involves a direct reaction to an external threat. However, the other statements involve responses to internal stimuli that are also important and valid responses to bodily needs.