Final answer:
The check valve is designed to quickly stop the flow of gas during a flashback, which is a safety measure to prevent the risk of explosions or fire.
Step-by-step explanation:
The device designed to quickly stop the flow of gas during a flashback is the check valve. A flashback is a dangerous condition that occurs when the flame from a torch moves back into the equipment's hoses, and potentially into the gas supply, causing a risk of explosion or fire. The check valve is installed in gas lines to prevent reverse flow of gas, which can contribute to this kind of accident. In comparison, a flowmeter measures the rate of gas flow, a reverse flow valve is designed to prevent the reverse flow of gases without extinguishing a flashback, and a pressure regulator controls the pressure of the gas supply.