"Extinction of a classically conditioned response may eventually occur when the stimulus is presented repeatedly without presentation of the unconditioned ."

Conditioned; stimulus

Explanation:

"Extinction of a classically conditioned response may eventually occur when the conditioned stimulus is presented repeatedly without presentation of the unconditioned stimulus."  Classical conditioning results from the association of an unconditioned stimulus to a conditioned stimulus in order to elicit a conditioned response. When the association of the unconditioned stimulus to the conditioned stimulus stops, classical conditioned response stops. For example, if a dog has been though to associate ringing of bells (conditioned stimulus) to the presentation of food (unconditioned stimulus) which results in salivation (conditioned response), continuous ringing of the bell without the presentation of food will result in the extinction of the classical conditioned response.