Which of the following indicates a warning was issued?

The indication that a warning was issued is represented by the issuance of a citation. A citation acts as a formal written notice documenting that a violation has occurred, which typically serves as a warning or a record of the infraction that may require further action or compliance. This formal note signals to the individual involved that there is a need to correct a behavior or adhere to a specified guideline, making it a clear representation of a warning being issued.

In contrast, a meal break, no action taken, and advice given do not signify that a warning has been communicated. A meal break is simply a pause in work without implications of behavioral correction. "No action taken" suggests that there were no violations or issues to address at that time. Similarly, while advice given can indicate an effort to guide or inform, it does not carry the formal weight or intent of a warning like a citation does.