The most common compound used in a fire extinguisher is ______________.

The most common compound used in a fire extinguisher is carbon dioxide (CO₂)

 Carbon dioxide does not support combustion. It is cold when it comes out of the fire extinguisher, so it also cools the fuel. The rest of the compounds in other options do not affect and stop the combustion process as much, so they cannot be used in the fire extinguisher.