Important Note: This question has been created from limited feedback and we are not precisely sure of the wording or the available answer options. We would very much appreciate further feedback should you see this question in your exams. Thank you!

Refer to figure.
This question asks specifically about the components related to aircraft pressurisation. As such, we are not concerned about the aircraft air conditioning air cycle system components which are used to cool and dehumidify compressed engine air to bring it to a suitable temperature for the cabin. The pressurisation system consists of the following elements:

• Outflow valve / flow control valve - This is what regulates cabin pressure under normal operation. The PACKS continuously insert pressurised air into the cabin, but the outflow valve opens and closes to vary the amount of air exiting the cabin, in order to increase or decrease the differential pressure (the pressure difference between the cabin and ambient air).
• (Positive) Pressure relief valve - Should the cabin pressure become excessively high, opening the outflow valve fully may not be sufficient to relieve the cabin pressure. This could lead to overstressing the airframe. The pressure relief valve acts to release excess cabin pressure to the atmosphere if it detects an excessive cabin pressure.
• Negative pressure / Inwards relief valve - The aircraft structure is designed to withstand positive pressure loads as the cabin is pressurised repeatedly during high altitude flights. The aircraft skin is not optimised for a negative pressure, where the internal pressure is lower than the outside pressure. A negative pressure relief valve is a simple device designed to relieve an excessive negative cabin pressure.

The other components listed (Heat exchanger, water separator, mixing valve) are all related to the air cycle machine.