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 be best explained?
the conditioned stimulus

The conditioned stimulus  

Explanation:

The conditioned stimulus is a neutral stimulus before conditioning in classical conditioning. It becomes associated with the unconditioned stimulus. This unconditioned stimulus triggered this stimulus. Ivan Pavlov was the psychologist who proposed classical conditioning.  

Thus here in the above context when Karen eating at the airport had to get nauseous in flight. Next time she feels nauseous when going to a restaurant at the airport. She was conditioned by the smell of that food in restaurant.

rain should be the answer.