What type of oxygen system operates only when the user inhales?

The correct choice, Diluter Demand, refers to an oxygen system that is designed to deliver supplemental oxygen only when the user inhales. This system utilizes a demand valve mechanism, which opens to allow oxygen flow during inhalation and shuts off during exhalation. This function ensures that an adequate supply of oxygen is provided exactly when it is needed, thus conserving oxygen and increasing efficiency.

In comparison, the other systems operate differently. Pressure Demand systems provide oxygen under pressure at all times to ensure that it is available regardless of the user’s breathing pattern, making it less reliant on the act of inhalation alone. Continuous Flow Demand systems release a constant flow of oxygen, which is not dependent on breathing efforts, delivering oxygen continuously regardless of inhalation or exhalation. Electrical Pulse Demand systems, while not typically a standard type in aviation, would suggest a mechanism that might operate based on electronic sensors to detect breathing, but it's not the conventional setup for general aviation or commercial applications. Thus, the Diluter Demand system specifically caters to the inhalation requirement, making it the most suitable and correct choice for the question.