What type of valve is activated based on fuel gas flow rate?

The correct choice is the excess flow valve because it is specifically designed to automatically close when the flow rate exceeds a predetermined set point. This function is crucial for safety in systems that utilize fuel gas, as it helps to prevent excessive gas flow that could lead to dangerous conditions, such as explosions or leaks.

The excess flow valve plays a critical role in ensuring that if a pipeline breaks or if there is a malfunction leading to unregulated gas flow, the valve will respond by shutting off the gas supply to minimize risk and enhance safety. This automatic activation based on flow rate is what distinguishes it from other types of valves.

While a pressure relief valve is designed to operate based on pressure levels to prevent overpressure situations, and a shut-off valve is manually operated to stop flow, neither is primarily concerned with monitoring or reacting to flow rates. A fixed flow valve maintains a specific flow rate rather than providing safety shut-off based on flow increases, making the excess flow valve the most suitable answer for this context.