The correct answer is C) shut off the leak. The reasoning behind this is that the first priority in extinguishing an LFG (liquefied flammable gas) fire is to stop the source of the fuel. Shutting off the leak will cut off the supply of gas, which is the root cause of the fire. This is the most effective and safest initial action, as it prevents the fire from spreading and allows for safer firefighting efforts. The other options are not as effective. Using a chemical foam extinguisher may temporarily smother the flames, but it does not address the underlying leak. Sweeping the flames with water spray does not stop the gas supply. Calling the fire department is important, but should be a secondary action after shutting off the leak.