Final answer:
The Roman achievement that helped Christianity spread was their extensive roadways, which enabled efficient travel and communication throughout the empire. Option C is the correct answer.
Step-by-step explanation:
Among the many engineering feats of the Roman Empire, the construction of extensive roadways was particularly significant in aiding the spread of Christianity. These roads enabled not only the rapid movement of the Roman military and facilitated efficient administrative control but also provided a network for the exchange of goods, ideas, and, notably, the dissemination of new religions such as Christianity. Thanks to this system of well-constructed roads stretching vast distances, Christian missionaries and adherents could travel and spread their message with relative ease throughout the empire.