An animal that hides under a rock to avoid predators is responding to a(n) stimulus.

The correct answer for the fill in the blank is- External stimulus.  

A stimulus is any change in the internal or external environment that triggers a particular response in the individual.

External stimulus is the one that an individual receives from external environment or surrounding and gives a specific response against it.

In this question, predator is the external stimulus for the animal and hiding under the rocks is the response triggered by the individual against it.

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