The correct answer is option 4, Acetylene. Gas welding is a process that uses heat from a gas flame to join two pieces of metal together. Acetylene is commonly used as a fuel in gas welding because of its high heat output.

Option 1, LPG (liquefied petroleum gas), is mainly composed of propane and butane. While LPG is commonly used in heating and cooking, it is not typically used as a fuel in gas welding because it does not produce a high enough temperature for the welding process.

Option 2, Ethylene, is a flammable gas that is primarily used in the chemical industry for the production of various compounds. It is not commonly used as a fuel in gas welding.

Option 3, Methane, is the main component of natural gas. It is commonly used for heating and cooking, but it is not typically used as a fuel in gas welding as it does not generate enough heat for the welding process.

Therefore, the correct answer is option 4, Acetylene, which is commonly used as a fuel in gas welding due to its high heat output.