The correct answer is C) the poor and oppressed.
In its early years, Christianity spread throughout the Roman Empire because its teaching appealed especially to the poor and oppressed.
After Roman Emperor established the Christian religion as the official one through the Roman Empire, the doctrine was easily accepted by the poor and oppressed people because they saw many similarities between the life of Jesus of Nazareth and their own lives of suffering and pain.
Indeed, there was not a real opposition to Christianity and that is why, with the acceptance of the people, the Roman Empire could establish better control of its territories through the presence and teachings of religion.