What must be posted to prevent entry into danger areas that cannot be observed?

The correct answer addresses the need to have a physical presence in potentially dangerous areas. Road guards serve a crucial role in ensuring safety by controlling access to these danger zones. Their presence allows for real-time monitoring of entry into areas that might not be visually observable, thereby helping to mitigate risk effectively.

While warning signs serve as essential indicators of hazards, they may not physically prevent entry into a dangerous area. Safety barriers could also be effective in blocking access, but without human oversight, they might not be sufficient alone, especially in complex or active environments. Training restrictions focus on informing personnel about the dangers, but they rely on individuals adhering to those guidelines, which doesn't provide the immediate, proactive management of risk that road guards do.