Final answer:
Option (A), Christianity, a monotheistic religion based on the teachings of Jesus of Nazareth, promised salvation to all individuals and was particularly appealing to the poor and enslaved in ancient Rome.
Step-by-step explanation:
Early Christianity and the Roman Empire
The monotheistic religion that hastened the decline of the Roman Empire and appealed to the poor and enslaved is Christianity. It's rooted in the teachings of Jesus of Nazareth and emphasized the spiritual equality of all human beings. The faith grew in popularity among slaves and common people due to its universal promise of salvation and strict monotheism, a stark contrast to Roman pagan practices.
Despite initial persecution by Roman authorities, Christianity's inclusive message and active evangelism led to a wide and rapid spread throughout the Empire. Its expansion and eventual official adoption by the state played a role in transitioning Roman society and politics.
Christianity was distinct from other mystery religions of the time in its refusal to worship Roman deities, the emperor, or participate in civic cults. Its monotheistic stance and promotion of love and compassion for all, regardless of social status, were revolutionary ideas in Roman society. This, along with its Jewish roots and the teachings of Jesus Christ, would lead Christianity to become the Empire's official religion by the end of the 4th century CE, marking a significant cultural and religious shift within the Empire.