Ever since Allison vomited all over her boyfriend after she ate a chicken salad sandwich that had been sitting in the sun all day, she becomes nauseous at the smell or taste of chicken salad. In this example, the conditioned stimulus is and the conditioned response is nausea.

the smell or taste of chicken salad

Explanation:

A conditioned stimulus is the one which was previously a neutral stimulus but hen it gets paired with the unconditioned stimulus it leads to a conditioned response. The learned response which is produced in response to a neutral stimulus is the conditioned response. In the given condition the smell or taste of a chicken salad causes Allison to feel nauseated and hence, the taste or smell of a chicken salad is the conditioned stimulus which generates a conditioned response which in this case is nausea.

The answer is a if u read the definition it makes sense