Which of the following valves should not be turned off during the extinguishing process of an oxyacetylene torch?

The oxygen gas valve should not be turned off during the extinguishing process of an oxyacetylene torch because it is essential for ensuring complete combustion when the flame is being extinguished. Proper extinguishing involves a controlled process to prevent backfire or flashback. Turning off the acetylene first can lead to a situation where the flame becomes unstable and may continue to burn with insufficient fuel, which can be unsafe.

By leaving the oxygen valve open during the extinguishing procedure, it allows for a more controlled and safer release of the remaining acetylene gas, ensuring that no unburned gas accumulates which could potentially lead to an explosion hazard. This approach maintains an adequate supply of oxygen for combustion until the acetylene is fully depleted, promoting a cleaner and safer extinguishment.