What action causes the brakes to activate in an emergency?

The option that correctly describes the action that causes the brakes to activate in an emergency is the activation of the passenger emergency handle unit. This mechanism is specifically designed for situations where immediate stopping of the vehicle or system is essential for safety. When the handle is engaged, it signals the braking system to initiate a halt, ensuring that the vehicle comes to a stop as quickly as possible.

In many emergency scenarios, this action serves as a direct interaction designed for non-technical personnel to trigger a rapid response, thus maximizing safety for passengers and crew members alike. The system is built to function under critical conditions, making it a reliable tool in emergencies and a key component of the safety protocols in transportation systems.