The correct answer is: d. the pope's crowning of Charlemagne as emperor of Rome.
This event symbolized a significant political and religious shift, which increased tensions between the papacy and the Byzantine Empire, as the Byzantine emperors considered themselves the legitimate rulers of the Roman Empire. The crowning of Charlemagne by Pope Leo III in 800 AD underscored the growing divide between the Roman Catholic and Eastern Orthodox churches and contributed to the eventual Great Schism in 1054.