Answer:
unconditioned stimulus
Explanation:
In classical conditioning, the unconditioned stimulus is the stimulus that triggers an unconditioned response. In this case, Brad Pitt's face is the unconditioned stimulus and the audience being affected by him is the unconditioned response.
The unconditioned stimulus occurs naturally and doesn't have to be related to prior learning experiences. On the other hand, the conditioned stimulus (Chanel No. 5 perfume) is associated with Brad Pitt's face and should eventually produce the same response.