Question

Most learning involves the process of association. with classical conditioning, an organism comes to associate:

Answer 1

I believe the answer is Two stimuli.
These two stimuli are usually different, but our mind make a same response for these stimuli as if  they're the same because they're associated with one another.
For example, lightning is actually different from thunder. But since they often come together, we associate lightning with thunder and we would automatically response to thunder when we see the lightning.