The correct answer is conditioned stimulus.
The most famous explanation of a conditioned stimulus is from Ivan Pavlov's classical conditioning study. In his study a bell was repeatedly rang (neutral stimulus) before dogs were presented with food (unconditioned stimulus) that made them salivate in response (unconditional response). After repeated pairing of the neutral stimulus with the unconditioned stimulus, the sound of the bell alone elicited a response of salivation in dogs. In this this instance, the conditioned stimulus is the sound of the bell.
