When in an unsafe condition for flight, which door(s) will show a red indicator on the synoptic page?

The correct choice is the main cabin door. When in an unsafe condition for flight, it is crucial to indicate which doors may pose a threat to safety. The main cabin door is specifically monitored for secure closure, as its status is critical to pressurization and overall cabin safety during flight.

If the main cabin door is not properly secured, it can compromise the cabin environment, potentially leading to a loss of cabin pressure or other serious issues during flight. The red indicator on the synoptic page serves as an alert for flight crew members, ensuring they can take appropriate action before takeoff.

In contrast, other doors, such as the emergency exit doors, have different monitoring systems in place, and while they are important for safety, their indicators may be managed separately from the main cabin door's system. The red indicator specifically focuses on the main cabin door's status as it has the most direct impact on the operational state of the aircraft.