For several weeks, Allen had to clean the restroom at the restaurant where he worked. The task always made him nauseated. He has since gone on to another job, but still cannot walk by the door to a restroom without becoming slightly queasy. For Allen, the door to the restroom has become a(n) .

conditioned stimulus  

Explanation:  

In psychology, the conditioned stimulus is one of the influencing factors in the classical conditioning theory which was proposed by Ivan Pavlov in an attempt to experiment on dogs.

Conditioned stimulus refers to a formerly neutral stimulus which is responsible for triggering a particular conditioned response after getting connected with an unconditioned stimulus.

In reference to the question above, for Allen, the door to the restroom has become a conditioned stimulus.

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