If someone shows erratic behavior and mood swings at work, which action is appropriate?

Reporting the behavior to a manager is appropriate in this situation because it ensures that the issue is addressed by someone in a position of authority who can take appropriate action. Erratic behavior and mood swings can be signs of underlying issues that may affect the well-being of the individual and the work environment. By bringing this concern to a manager's attention, it allows for a formal assessment of the situation and the implementation of any necessary support or intervention. This approach prioritizes the safety and mental health of all employees and fosters a supportive workplace culture.

Monitoring the situation may be seen as passive and may not lead to any resolution. Discussing it with co-workers can lead to gossip or misunderstandings, while ignoring the behavior could escalate the issue and create a more significant problem later on. Therefore, reporting to a manager is the most responsible and proactive action.