Which of the following is a common source of foodborne illness in food service?

Improperly washed fruits and vegetables are indeed a common source of foodborne illness in food service. When fruits and vegetables are not adequately washed, they can retain harmful pathogens, such as bacteria, viruses, and pesticides, which can lead to health risks when consumed. Contaminated produce is often linked to outbreaks of foodborne illnesses, highlighting the importance of proper cleaning and sanitation practices in food service settings.

Other options, while they may involve some potential risks, do not present the same level of direct concern for foodborne illness. Using frozen ingredients, for instance, can be a safe practice as freezing can inhibit the growth of pathogens. Cooking at low temperatures, when done appropriately, can also be a part of safe cooking practices if it adheres to established guidelines, such as sous vide methods that ensure food is cooked for an adequate amount of time to kill harmful organisms. Serving prepackaged foods typically reduces the risk of foodborne illness since these items are often processed and handled in controlled environments before reaching the consumer.