Which of the following must a security guard do when witnessing a crime?

The appropriate action for a security guard upon witnessing a crime is to call law enforcement. This is essential because it ensures that trained professionals can respond to the situation appropriately. Security guards are not law enforcement officers and are therefore not equipped or authorized to intervene physically in most situations. Their main responsibility is to observe, report, and ensure the safety of the environment they are monitoring. By contacting law enforcement, a security guard facilitates a timely and professional response to the crime, helping to maintain order and safety in the vicinity. Ignoring the crime or only reporting it if it escalates could lead to further harm or danger, while physical intervention could escalate the situation unnecessarily and put the guard at risk. Therefore, calling the authorities is the most responsible and effective course of action.