The correct answer is A. acetylene in oxygen.
Acetylene is a highly flammable gas that burns with a very hot flame. When it is mixed with oxygen, the flame can reach temperatures of up to 3,500 degrees Celsius. This is hot enough to melt most metals, which is why acetylene is often used in welding.
LPG (liquefied petroleum gas) is a mixture of propane and butane. It is also flammable, but it does not burn as hot as acetylene. The flame temperature of LPG is typically around 2,000 degrees Celsius.
Methane is the main component of natural gas. It is also flammable, but it burns even cooler than LPG. The flame temperature of methane is typically around 1,500 degrees Celsius.
Nitrogen is an inert gas. It does not burn and does not support combustion. Therefore, it cannot be used to produce a high-temperature flame.
In conclusion, the correct answer is A. acetylene in oxygen.