Who crowned Charlemagne as Holy Roman Emperor on Christmas Day in 800?
Explanation
The answer is Pope Leo III. On Christmas Day in the year 800, Pope Leo III crowned Charlemagne as the Holy Roman Emperor. The ceremony took place in St. Peter's Basilica in Rome, symbolizing the alliance and support of the Church towards the Frankish king. This act legitimized Charlemagne's authority and solidified his position as the ruler of a vast empire, blending both secular and religious power under his rule.