The correct answer is the catholic church.
After Rome's fall, the people of Europe turned to catholic church for stability and guidance.
It is during this fall that catholic churches continued to grow because,
1. Christianity was being proclaimed for the religion of masses.
2. The churches were organized socially, economically, and politically.
Roman empire split and Theodosius were the events which made christianity grew rapidly. Some of the reasons for the fall of Rome included,
1. Invasion by Barbarian tribes.
2. Overreliance on slave labour and economic troubles.
3. The rise of eastern empire.
Hope this helps :)
