Of the shellfish listed below, the ones most likely to cause illness are _______.

Oysters are known to be particularly associated with foodborne illnesses, primarily due to their filtering feeding habits. They can accumulate harmful bacteria, viruses, and toxins present in the water, which is especially problematic in warm waters. Pathogens such as Vibrio vulnificus and Norovirus are common culprits linked to oyster consumption. Because oysters are often eaten raw or undercooked, the risk of illness increases significantly.

In contrast, while crabs, shrimp, and lobsters can also carry bacteria if not handled or cooked properly, the incidence of illness linked to these shellfish is generally lower than that associated with oysters. Many cases of foodborne illness from crabs, shrimp, and lobsters are related to improper cooking and handling rather than the nature of the shellfish itself. Thus, the choice of oysters as the most likely shellfish to cause illness is justified based on their risk profile in the context of food safety.