Which of the following foods is most likely to be a source of Listeria monocytogenes?

Unpasteurized dairy products are highly associated with Listeria monocytogenes due to the potential for the bacteria to thrive in environments where raw milk is present. The Listeria bacteria can be found in soil and water, and when animals graze on contaminated pastures, the bacteria can colonize their udders. Consequently, any unpasteurized milk or products made from it (like certain cheeses) risks containing Listeria, especially when those products are not subjected to heat treatments that would eliminate the bacteria.

In contrast, while cooked meats and processed meats can pose a risk if handled improperly or contaminated post-cooking, the risk of Listeria specifically increases with foods that are not heat-treated. Canned vegetables generally have a low risk of harboring Listeria, as the canning process involves high heat to kill bacteria. Thus, unpasteurized dairy products stand out as a significant source of Listeria monocytogenes, which is why this answer is the most accurate.