The function of a thermocouple in a gas appliance is to:

The function of a thermocouple in a gas appliance is primarily to detect flame. A thermocouple is a sensor made of two different metals joined at one end, which generates a small voltage when heated. In the context of gas appliances, it is typically used to monitor the presence of a flame. When a flame is present, the thermocouple heats up and produces a voltage that helps keep the gas valve open, allowing for the continuous flow of gas to the burner. If the flame goes out, the thermocouple cools down, ceasing voltage production, which signals the gas control system to shut off the gas supply, thereby enhancing safety.

The other options do not accurately describe the function of a thermocouple. Controlling temperature relates more to thermostats or temperature control systems, while measuring pressure pertains to pressure sensors or gauges. Regulating airflow typically involves dampers or fans, not a thermocouple's function. Thus, the ability of a thermocouple to detect flame is crucial for the safe operation of gas appliances.