Which component of the CRU-120/P provides overpressure protection when the vest is not worn?

The pressure relief valve is the correct answer because its primary function is to prevent overpressure situations within the system when the vest is not worn. This component activates to release excess pressure that may build up in the air supply system, ensuring that the system remains within safe operating limits and protecting the integrity and functionality of the equipment. In the context of the CRU-120/P, which is an air supply system used in various flight equipment vests, the pressure relief valve is crucial for maintaining safety, especially when the vest is not actively being used and when pressure levels may fluctuate.

The other components, while important for the overall functionality of the system, have different roles: the flow regulator manages the rate of airflow, the manual shutoff valve is used to control the overall supply of air to the vest, and the emergency bypass allows for rapid access to air in emergency situations. However, none of these components are specifically designed to address the issue of overpressure protection when the vest is not being worn, which is why the pressure relief valve is the correct choice.