The answer is unconditioned stimulus or us. In classical conditioning, it is an unlearned response that happens unsurprisingly in response to the unconditioned stimulus. For instance, the man that yanna used to date is the unconditioned stimulus, she remembers the smell of lime in response to the person is the unconditioned response.
The conditioned response here is the scent of lime. A conditioned response was a neutral stimulus before, then after becoming linked with the unconditioned stimulus, it triggered this kind of response. In this case the scent was nothing but after sometime, it activated the feeling of arousal.