The fitting which is most critical in stopping a boiler explosion is:

The fitting that is most critical in stopping a boiler explosion is the safety valves. Safety valves are designed to automatically release excess pressure from the boiler system when the pressure exceeds a predetermined limit. This is crucial because if too much pressure builds up inside the boiler, it can lead to catastrophic failures or explosions.

By ensuring that any excess pressure is vented safely, safety valves help maintain the overall integrity of the boiler. They act as a last line of defense against pressure issues that could lead to an explosion, which makes them essential for safe boiler operation. In contrast, while low water cutoffs and feed water control valves contribute to safe boiler functioning by preventing low water conditions and controlling water input, they do not directly mitigate the risk of explosion due to excessive pressure. Automatic cutoffs are important for operational control, but they lack the specific pressure relieving function that safety valves provide.