What type of air from the pneumatic system is used to maintain the desired zone temperature?

The pneumatic system is designed to provide conditioned air for maintaining the desired temperature in different zones of the aircraft. The air referred to in the context of zone temperature control specifically pertains to the air that is directed into the various compartments of the aircraft to ensure comfort for passengers and crew.

In this context, the correct term associated with managing the temperature in the zones is 'trim air.’ Trim air refers to the air that is mixed with the conditioned air coming from the air conditioning system to fine-tune or adjust the temperature to the specified settings for each zone. This allows for individual comfort adjustments without impacting the overall performance or temperature of the air conditioning system.

The other options pertain to different types of air but do not specifically relate to the function of temperature maintenance in the zones. Internal air generally refers to air already present within the aircraft cabin, external air usually is that coming from outside the aircraft, and zone air does not specifically denote the adjusted air used for maintaining precise temperature control.