Final answer:
Christianity experienced striking success in urban settings within the eastern Mediterranean. The religion's communal practices and appeal across social classes aided its growth in these densely populated areas, despite periods of persecution.
Step-by-step explanation:
Christianity was most strikingly successful in urban settings (D). The urban centers of the Roman Empire, particularly those in the eastern Mediterranean, were crucial for the growth of Christianity. These cities provided a network for the spread of religious ideas and the construction of important Christian structures like monasteries and churches. The faith appealed to a wide array of urban dwellers, from the poor to the socially outcast, due to its radical teachings of universalism and the spiritual equality of all human beings. Despite facing periods of persecution, the Christian religion's acceptance by the government, particularly after Emperor Constantine's reign, fostered a transformative influence throughout the empire. The communal practices of Christians, such as communal feasting, addressment of one another as "brother" and "sister," and the initiation rite of baptism, further solidified their collective identity within these urban environments.