To solve this problem, we need to identify the psychological phenomenon that Stanley is experiencing due to his association between nausea and his favorite grilled cheese sandwich.

#### Solution By Steps

***Step 1: Identify the psychological phenomenon***

The phenomenon at play here is classical conditioning, a type of learning where a neutral stimulus (the grilled cheese sandwich) becomes associated with a response (nausea) due to repeated pairing with an unconditioned stimulus (chemotherapy-induced nausea).

***Step 2: Define classical conditioning***

Classical conditioning involves learning by association. It was first described by Ivan Pavlov, where a neutral stimulus (like a bell) is repeatedly paired with an unconditioned stimulus (like food) that naturally and automatically triggers a response (salivation in dogs). Over time, the neutral stimulus alone can elicit the response.

***Step 3: Apply classical conditioning to Stanley's situation***

In Stanley's case, the grilled cheese sandwich (neutral stimulus) was repeatedly experienced during chemotherapy (unconditioned stimulus), which naturally caused nausea (unconditioned response). Over time, the grilled cheese sandwich alone (now a conditioned stimulus) elicits nausea (now a conditioned response).

#### Final Answer

Stanley’s nauseous reaction to a grilled cheese sandwich is the conditioned response.