In classical conditioning a stimulus is any event or situation that

evokes a response.

Explanation:

Classical conditioning: The term classical conditioning is proposed by a psychologist named Ivan Pavlov after experimenting with dogs. According to him, classical conditioning is described as the behavior that is being learned through a repetitive association or exposure between the stimulus and the response.

Stimulus: Here, in the process of classical conditioning the stimulus works as an event or situation that evokes or leads to a specific response. The response to any stimulus will depend on the content of the stimulus. Every stimulus will have a different response.

In the question above, in classical conditioning, a stimulus is any event or situation that evokes a response.