Nclassical conditioning, the is an unlearned reaction to a given stimulus. for example, if you have an allergy to pollen and sneeze, sneezing is an unlearned reaction to the pollen (i. e., the stimulus).

The correct answer is unconditioned response.
According to Ivan Pavlov, the psychologist who came up with classical conditioning, an unconditioned response is something that happens naturally as a result of an unconditioned stimulus. For example, when you smell food (unconditioned stimulus) you might start to feel hungry (unconditioned response) - that's not something you learn. 

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