Final answer:
The 'L' in the FDA's ALERT system advises the person in charge to monitor the security of the food supply by looking for any unusual activities that could compromise food safety.
Step-by-step explanation:
The FDA's ALERT system is a food defense plan that guides establishments on how to protect the food supply against intentional contamination. The acronym ALERT stands for Assure, Look, Employees, Reports, and Threat. The "L" in ALERT instructs the person in charge to Look by monitoring the security of the products in the establishment. This involves keeping an eye out for anything out of the ordinary that could potentially harm the food supply, such as unauthorized personnel in restricted areas, unexplained alterations to food products, or signs of tampering.