Darrel was dancing with his new girlfriend at an Elvis tribute. When the band started playing, "Can't Help Falling in Love with You," his girlfriend gave him a long, passionate kiss, which Darrel found very enjoyable. Now Darrel finds that every time he hears "Can't Help Falling in Love with You" on the radio, he becomes mildly excited. In this example, the long, passionate kiss is

a. a conditioned stimulus.
b. a conditioned response.
c. an unconditioned response.
d. an unconditioned stimulus.

In classical conditioning, first we have a stimulus that triggers a natural response by itself. This stimulus is called the unconditioned stimulus (US) and the natural response triggered is the unconditioned response (UR).

Later, we start presenting a stimulus that doesn't trigger any response by itself and we pair this stimulus with the unconditioned stimulus. This new stimulus is called the conditioned stimulus (CS), soon, after some repetitions, the response will start to show up in presence of the CS, and now the response is called conditioned response (CR).

In this example, Darrel was dancing and when one song started playing she gave him a kiss. The kiss would be the unconditioned stimulus that triggers the natural response of making him excited (unconditioned response). He paired this stimulus with the song (conditioned stimulus) and now every time he listens to the song (CS) he becomes mildly excited (conditioned response). Thus, in this example, the kiss is a unconditioned stimulus.