What will signal the men when it's time to go?

The correct answer is based on the context of how various signals are communicated. In this case, "Alert" refers to a type of signal that can notify or call attention to an event, indicating that it is time for the men to go. Alerts can take various forms, including visual cues or auditory signals, but fundamentally, the concept of an “alert” serves the purpose of informing individuals that they need to take a specific action, such as leaving or proceeding.

The other options may suggest a signal, but they do not encapsulate the broader meaning of alerting someone to action as effectively. A light, sound, or bell all serve as potential means of signaling, but they are more specific examples of alerts rather than encompassing the overall concept of notifying someone. Thus, "alert" succinctly conveys the idea of a signal that triggers a response to depart.