The correct answer is D. It united many different European kingdoms under a shared religious culture.
Indeed, with the fall of the Roman Empire, the Roman way of life was not eliminated; it actually subsisted for a long time after the Empire fell and incorporated all the barbarian invader’s customs and ruling structures.
Catholic authorities had to carefully negotiate precarious agreements with the new barbarian rulers and to keep a balance between their paganism and Christian beliefs.
Eventually each one of these new barbarian kingdoms would be converted to Christianity by the work of Christian missionaries. One result of this was that the Christian faith spread to areas where Roman rule had never existed, like Ireland, the Northern Germanic kingdoms and Scandinavia. Before Protestantism, this united many different European kingdoms with a share Christian religious culture.