Answer:
unconditioned stimulus.
Explanation:
Unconditioned stimulus: In psychology, the term "unconditioned stimulus" or UCS or US is referred to as one of the parts in the classical conditioning theory and is defined as a stimulus that generally triggers a specific response effortlessly whenever it is being presented in the absence of any former training and the given response is referred to as the unconditioned response or UCR or UR.
In the question above, the nausea-inducing agent is the unconditioned stimulus.