Final answer:
Pope Leo III rewarded Charlemagne by crowning him Emperor of the Romans/Holy Roman Emperor, strengthening Charlemagne's power and influence in his empire.
Step-by-step explanation:
Pope Leo III rewarded Charlemagne by crowning him "Emperor of the Romans"/Holy Roman Emperor. This event took place on December 25, 800. It symbolized a close relationship between the Church and the emperor, and it strengthened Charlemagne's power and influence in his empire.
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