When must all flight crew wear oxygen masks with EMERGENCY selected?

The correct scenario for when all flight crew must wear oxygen masks with EMERGENCY selected is during a cabin depressurization. This requirement arises because a rapid depressurization can expose cabin crew and passengers to insufficient oxygen levels. In such cases, wearing an oxygen mask with the emergency setting ensures that crew members receive a higher concentration of oxygen, which is crucial for maintaining consciousness and effectively managing the aircraft's operation during an emergency.

In terms of the other options, while wearing oxygen masks can be important in various situations (like fire or high altitude), the specific requirement for all flight crew to wear oxygen masks with EMERGENCY selected is primarily triggered by cabin depressurization events, as this poses an immediate risk to their ability to function and respond effectively in an emergency.