Answer:
the growth and expansion of the religion
Explanation:
1. a decrease in Mongol invasions: this option is not correct because Mongol did not invade main Europe
2. a population explosion caused by the Neolithic Revolution: this option is not correct because the Neolithic period was about 10,000 BCE, the Romans and Frankish have not been defined or in existence then, nor was there Christianity during this period.
3. an increase in the influence of prior civilizations on Mayan society: this is not correct because Mayan civilization was not in Europe but southeast Mexico and Central America, hence there was no connection
4. The growth and expansion of the religion: this option is CORRECT because the Romans and Frankish kingdoms were known to be majority Christians and helped spread the religion across the world.
