What button's pressure must be maintained to keep the doors open in Phase 2?

To keep the doors open in Phase 2 operation of an elevator, the pressure on the door open button must be maintained. Phase 2 operation typically refers to a situation where the elevator is under fire service mode, allowing firefighters to have control over the elevator's functions while ensuring the doors remain open for safe passage. The door open button, when continuously pressed, sends a signal to the elevator’s control system to hold the doors in an open position, thereby facilitating emergency access.

In this context, other buttons like the door close button would conversely command the doors to shut, while the emergency button is designated for activating emergency protocols. The safety button serves various safety interlocks but does not specifically function to maintain the doors in an open state. Therefore, the correct and most relevant choice in this scenario is the door open button.