Three hours after eating at an airport fast-food restaurant, Karen got extremely nauseous while in flight. The next time she was in the airport and walked by that restaurant, she felt a wave of nausea. How can this best be explained?

the conditioned stimulus

Explanation:

A conditioned stimulus in classical conditioning is a stimulus which works in association with an unconditioned stimulus to brings about a conditioned response. The conditioned stimulus in this scenario is the sight of that restaurant and the conditioned response is the wave of nausea she feels.