Which component is essential for preventing gas flow when an appliance is off?

The component that is essential for preventing gas flow when an appliance is off is the shutoff valve. This valve is specifically designed to control the flow of gas to an appliance and can be manually or automatically operated. When the appliance is turned off, the shutoff valve can be closed to effectively stop the flow of gas, ensuring safety and preventing the risk of leaks.

The pressure regulator, while important for maintaining appropriate gas pressure for appliances during operation, does not directly serve the function of stopping gas flow when the appliance is not in use. Similarly, thermostatic control systems are used to manage temperature settings and operation timing but do not control gas flow when appliances are turned off. The gas meter is primarily used to measure gas consumption for billing and is not designed to cut off gas flow to appliances when they are inactive. Hence, the shutoff valve is the crucial component for ensuring that gas does not flow when an appliance is not operational, making it the correct answer.