Which of the following is a main rule during a lockdown situation?

In a lockdown situation, the primary focus is on preserving the safety and security of everyone on board. Keeping the flight deck door closed and secure is crucial because it helps to protect the flight crew from unauthorized access and potential threats. By not opening the flight deck door during a lockdown, the integrity of the secure area is maintained, allowing the crew to continue operating the aircraft safely.

Other guidelines, such as keeping cabin crew silent, having passengers remain seated, or restricting food and beverage service, may also be situationally relevant but do not carry the same level of critical importance as securing the flight deck. The emphasis on not opening the flight deck door is essential in crisis management protocols and ensures that the potential for escalating a threat is minimized during a lockdown situation.