The correct answer is - B) Mongols.
The Western Roman Empire never came into a situation to be in a war against the Mongols. The Mongols invaded parts of Europe few centuries after the Western Roman Empire was gone from the map. The other three mentioned, the Huns, Saxons, and Vandals, all contributed to the demise of the Western Roman Empire, and in general, the Huns are considered to have made the biggest damage to it, even though they never managed to take down Rome. The Huns exhausted the Romans in military and economic aspects, and that opened the door for the next invading forces to have it much easier with the Romans.
