Which of the following conditions is necessary for a foodborne illness to be classified as an outbreak?

For a foodborne illness to be classified as an outbreak, it is essential that two or more people experience the same illness after consuming the same food. This requirement indicates a shared exposure and a common cause, which is fundamental in distinguishing an outbreak from isolated cases. The presence of similar symptoms among the affected individuals supports the conclusion that they were likely affected by the same pathogen or toxin, making Option C the correct choice.

In contrast, other conditions presented do not meet the standard criteria for an outbreak. For instance, one person affected by the same food does not establish a pattern of illness that suggests a widespread issue, while two or more people with different symptoms may not indicate a related outbreak if the illness types are varied. Furthermore, if a common food source is undetermined, it complicates the identification of the source but does not inherently define the occurrence of an outbreak without the clear link of illness and symptoms among multiple people.