What is a common feature of the oxygen system found in the flight deck?

The correct choice highlights that the diluter-demand system is a common feature of the oxygen system found in flight decks. This type of system is designed to provide pilots and crew with an adequate supply of oxygen based on their needs.

In a diluter-demand system, oxygen is delivered only when the user inhales, which is highly efficient in conserving the oxygen supply. The system mixes atmospheric air with the supplied oxygen, allowing for an adjustable flow that matches altitude and cabin pressure conditions. This is particularly advantageous in the flight deck environment, where crew members may operate at varying altitudes and may not require a continuous supply of oxygen at all times.

In comparison, other systems mentioned, such as the continuous flow system, deliver a constant supply of oxygen regardless of the user's inhalation, which can lead to waste. The pressure-demand system is also widely used but is typically found in scenarios requiring higher altitudes where supplemental oxygen needs must be delivered under pressure. The fixed oxygen system is less common and usually found in scenarios where a set oxygen supply is permanently installed without the need for user control. Therefore, the diluter-demand system stands out as a practical and efficient method suitable for the dynamic needs of a flight deck environment.