Carson used to enjoy lime sherbet, and when he was in mexico he tried frozen lime margaritas. after his fourth margarita carson became extremely ill. now he finds that even the sight of lime sherbet in a bowl can make him feel queasy. in this example, the unconditioned stimulus is

I believe the answer is: the lime margarita that Carson consumed

unconditioned stimulus refers to the triggers that naturally creates the response. In the scenario above, the response of being ill is created by the lime margarita, not the lime sherbet. Therefore, lime margarita is the natural trigger of the Ill response while the nausea for seeing lime sherbet is a modified response.