A listed __________ shall be installed at the discharge end of the transfer hose at vehicle fuel dispensers.

The inclusion of a quick-acting shutoff valve at the discharge end of the transfer hose at vehicle fuel dispensers is critical for safety and efficiency. This type of valve is designed to close rapidly, ensuring that the flow of propane can be stopped almost immediately in case of an emergency or malfunction. This feature can significantly reduce the risk of leaks or spills, which are crucial considerations given the flammability of propane.

In fuel handling scenarios, where quick accessibility to shut off the flow is vital, this valve acts as a safety mechanism, enhancing the overall safety of the dispensing process. Additionally, adhering to safety regulations often mandates the installation of such valves to mitigate potential hazards that can arise during fuel transfers.

The other options, while they play important roles in the overall safety and functionality of fuel systems, do not specifically address the necessity for immediate cessation of flow in the context described. The quick-release valve and transfer coupler are designed for different functions related to connection and disconnecting of hoses, and a pressure regulator serves to manage the flow and pressure of the propane rather than providing an immediate shutoff capability.