What must be the PA to all cabin crew during an attempt to intrude into the flight deck after shouting "Door! Door! Door!"?

The correct choice reflects the appropriate alert level to be communicated to all cabin crew when a threat is identified, specifically during a situation involving an attempted intrusion into the flight deck. When the phrase "Door! Door! Door!" is shouted, it indicates a potentially serious security breach that requires immediate and decisive action.

The designation of Level 4 indicates that the threat is imminent and could involve life-threatening circumstances, thereby warranting the highest level of response from the cabin crew. This protocol is put in place to ensure that all members are aware of the seriousness of the situation and can act swiftly to manage it effectively. It helps to ensure coordinated action and the prioritization of safety, especially in high-stress situations.

Levels of response are categorized to ensure that cabin crew members understand the severity of various situations. Lower levels, such as Level 1 or Level 2, would typically correspond to less critical issues that do not pose an immediate danger, thus they would not be appropriate in the context of a potential flight deck intrusion.