Which component prevents sudden loss of pressurization if the source is lost?

The correct component that prevents sudden loss of pressurization if the source is lost is the delivery air duct check valve. This check valve is critical because it allows airflow in one direction while preventing any backflow. In the case of a sudden loss of cabin pressure due to failure of the pressurization system, the check valve helps to maintain enough pressure by preventing outside air from entering the cabin through the ducts. This is essential for passenger safety and comfort, as a sudden loss of pressurization can lead to rapid decompression, which poses serious risks.

The other components mentioned contribute to different functions within the cabin atmosphere control system. The mixing valve, for example, is primarily responsible for regulating the mixture of heated and cooled air, thus controlling the temperature rather than pressurization stability. The negative pressure relief valve serves to equalize pressure differences and prevent structural damage but does not actively control pressurization during a failure. The expansion valve is involved in managing refrigerant flow in the cooling system, but it does not directly address the operational integrity of the cabin pressure in case of a source failure.