Which of the following is a primary source of air used for cabin pressurization?

Cabin pressurization in an aircraft is primarily achieved through the use of bleed air. This is air that is taken from the engine's compressor sections, which has been compressed and heated to a suitable temperature. The engines provide a continuous supply of this bleed air to maintain cabin pressure during flight.

Using bleed air is advantageous because it helps ensure that the cabin remains at a comfortable pressure and temperature while also oxidizing the air that supports combustion in the engines. This approach integrates seamlessly into the aircraft's overall systems, ensuring a consistent and reliable source for cabin pressurization.

The other options, while related to air management in the cabin, do not serve as the primary source of air used for this purpose. Recycled cabin air involves filtering and reintroducing air already inside the cabin, outside ambient air must be conditioned before being introduced, and relying solely on air from engines without specifying bleed air lacks precision in the context of cabin pressurization.