What must be installed in a CNG transfer hose to ensure safety during operation?

In the context of CNG transfer operations, the correct answer emphasizes the importance of a pull-away device. A pull-away device is a safety feature designed to disconnect the transfer hose from the fuel source automatically if there is an accidental separation during the fueling process. This mechanism helps prevent spills or leaks of compressed natural gas, which could pose significant hazards due to the flammable nature of the gas. By ensuring safe disconnection in case of an unintentional pull on the hose, it minimizes the risk of accidents involving both personnel and equipment.

While the other options serve essential roles in different aspects of CNG operations, they do not specifically address the immediate safety concern related to hose disconnection during transfer. An emergency shut-off valve is crucial for shutting down the gas supply in emergencies, but it doesn't prevent the initial hazards associated with a disconnected hose. A pressure relief valve is vital for managing pressure within the system, ensuring it does not exceed safe limits, yet it doesn’t address the risks directly tied to disconnection incidents. Lastly, a flow meter is important for measuring the volume of gas being transferred but does not contribute to safety in the event of a hose being pulled away. Thus, the pull-away device is critical for ensuring operational safety during CNG transfers