Final answer:
The role of the Catholic Church was the most unifying factor for Western Europeans during the medieval period, providing a common cultural and religious identity.
Step-by-step explanation:
The factor that most unified Western Europeans during the medieval period was C) The role of the Catholic Church. Despite the fragmentation of political power among various Germanic kingdoms and the decline of centralized authority with the fall of the Western Roman Empire, the Church provided a unifying cultural and religious force across Western Europe. It established a multinational bureaucracy that extended far beyond the reach of any feudal lord, offering a sense of community and shared religious identity to the diverse peoples of medieval Europe.