The correct answer is A) Mongols.
One of the groups that did not threaten Charlemagne's empire after his death, ultimately contributing to the development of feudalism as a means of defense were the Mongols.
In 813, Charlemagne crowned his son Louis the Pious, He was a co-emperor that substituted his father after his death in January 814. The Mongols decided not to invade Charlemagne's empire. They already had so much in their hands. The Mongols were founded by Genghis Kan in 1206. He controlled a vast empire: from the Caspian Sea to the China Sea; from Siberia to the Tibet.
The other options of the question were b) Vikings, c) Magyars, and d) Muslims.
