Final answer:
Cross-contamination occurs when a food handler prepping a salad stops to scratch an itch on her arm and then returns to making the salad. It can lead to the transfer of bacteria or pathogens, making the food unsafe to eat.
Step-by-step explanation:
The food becomes unsafe due to Cross-contamination when a food handler prepping a salad stops to scratch an itch on her arm and then returns to making the salad. Cross-contamination occurs when microbes are transferred from one food to another. If the handler's unwashed hands come into contact with the salad, it can lead to the transfer of bacteria or pathogens, making the food unsafe to eat.