What type of valve prevents the reverse flow of gases through the torch and hoses?

The correct choice is a check valve, as it is specifically designed to allow fluid or gas to flow in one direction while preventing backflow. In the context of welding, this function is crucial to ensure that the gases used, such as oxygen and acetylene, do not flow backward into the hoses or torch. This backflow could potentially lead to hazardous situations, including flashbacks or gas mixing that can create explosive mixtures.

Check valves achieve this by utilizing a mechanical mechanism that only opens under positive pressure from one direction, effectively blocking reverse flow. This is vital for maintaining safety in welding operations as it protects the equipment and the welder from the risks associated with backflow, which could also lead to contamination of gas mixtures.