The correct answer is C. Christianity
During the Middle Ages, which is a period of History that encompasses from 475 AD to 1453 AD, people of Europe were united through strong religious bonds provided by the Christian faith. In fact, in this region, there was no European identity at all, neither there were strong national identities. Most of the people were identified as Christian and felt linked to the Church above all, in spite of having many different languages. This sense of belonging to Christianity differentiated people in Europe from other pagan people that still existed and Muslims that were struggling in the borders of Christendom.
It was this shared feeling of identification which pushed Christian lords to follow the Pope's enactment to take the Holy Land during the Crusades in an attempt to extend the Church's dominion.
This union provided by Christianity started to erode with the beginning of modernity, the rise of absolute monarchy and national identity, and the Scientific Revolution that put religion into question.
