Final answer:
The correct answer is option a) Saint Paul.
Step-by-step explanation:
Among the individuals named, Saint Paul had the most significant role in the spread of Christianity after Jesus. As a relentless evangelist, Saint Paul's efforts were pivotal in the transformation of Christianity from a small Jewish sect into a global religion. Paul's missionary journeys and the establishment of Christian communities throughout the Greco-Roman world were crucial in broadening Christianity's reach to Gentile populations. His letters, or Epistles, addressed to these early Christian communities, became an integral part of the New Testament.
On the other hand, Constantine the Great also played a critical role, but his influence emerged later during the establishment of the religion's status within the Roman Empire. As the first Christian emperor, Constantine's conversion and subsequent support for the religion marked a significant turn in history, leading to the eventual dominance of Christianity in Europe and the creation of a Christian state. Although Constantine's actions were significant, they occurred after the initial spread of Christianity, which was primarily accomplished through the missionary work of individuals like Paul.