Answer:
In this example, the conditioned stimulus is the pizza.
Explanation:
The conditioned stimulus can be described as a stimulus which previously was normal. After becoming exposed to an unconditional event, the conditional stimulus starts to trigger a different response or approach.
For example, the scenario described in the question, eating or seeing pizza was normal for Joseph. After encountering an unconditional event, the pizza started to trigger an abnormal kind of response in Joseph. Hence, pizza is the conditional stimulus here.