The correct answer is C) Charlemagne.
The ruler that restored Pope Leo III to the Papal seat after the Romans drove him out was Charlemagne.
Let's remember that Pope Leo III crowned Charlemagne as the Holy Roman Emperor on December 25, 800 AD. Charlemagne was the King of the Franks. With those act, Charlemagne received religious support and legitimacy. So Charlemagne protected this Pope from his enemies. Pope Leo III was the Pope from December 795 until 816.