The correct answer is D) Secure the source of gas, then extinguish the fire. This is the correct approach because for a liquefied natural gas (LNG) fire, the priority is to stop the flow of gas first before attempting to extinguish the fire. Securing the source of gas will stop the continuous fuel supply, making it safer and more effective to then extinguish the remaining fire. Extinguishing the fire without first securing the gas source could allow the gas to continue leaking and re-ignite, creating an even more dangerous situation. The other options are incorrect because using only carbon dioxide (A) or dry chemical (C) may not be effective in fully extinguishing an LNG fire, and extinguishing the fire first without securing the gas source (B) does not address the underlying issue of the continuous fuel supply.