Answer: D. Both acted as unifying force in Europe.
Explanation:
After a rocky start in the Roman empire, the Catholic Church became the main religion of the Roman Empire such that most people subscribed to it. It united the various tribes under Roman control because they had a shared religion and a shared God.
The same thing continued in medieval Europe. With Christianity being the main religion of the continent, there were fewer conflicts because they worshipped the same God and could be criticized by the Pope if they acted too rashly.
