Most learning involves the process of associations. in classical conditioning, an organism learns to associate:

The right answer is Two Stimuli. Classical conditioning indicates that learning occurs when a previously neutral stimulus provokes a response to the presence of another stimulus due to the connection we make of both stimuli. For example, hear the bell that indicates the time of recess and feel energized. We learn to associate that sound with energy, although sometimes the bell does not indicate that it is time for recess.

C; skin, is the answer!