The correct answer is: Acetylene.
Acetylene is a gas that is used in gas welding. It is a colorless, flammable gas that is produced by the reaction of calcium carbide and water. Acetylene is used in gas welding because it has a high flame temperature and a low ignition temperature. This makes it ideal for welding metals that are difficult to weld with other methods.
LPG, ethylene, and methane are all gases that can be used in gas welding, but they are not as commonly used as acetylene. LPG is a mixture of propane and butane, which are both flammable gases. Ethylene is a colorless, flammable gas that is produced by the cracking of petroleum. Methane is a colorless, odorless, flammable gas that is the main component of natural gas.
Acetylene is the most commonly used fuel in gas welding because it has the highest flame temperature and the lowest ignition temperature of all the gases that can be used in gas welding. This makes it ideal for welding metals that are difficult to weld with other methods.