The correct option would be 'Slow growth followed by an explosion of growth'.
After the sacrifice and resurrection of Christ, the apostles had the mission to spread His words and teachings throughout the world. This process was marked firstly by the foundation of the Jerusalem Church (considered to be the first Christian church ever, with a few more than 100 jews and recent newcomers to Jerusalem (so-called Jewish Proselytes) followed by the dispersion of the apostles within the first decades.
The spread of Christians started slow because of the great resistance faced by the apostles, specially from the roman empire, which not until the rule of the Emperor Constantine I, also known as 'Constantine the Great' (who ruled from 306 to 337 AD) who was also the first roman emperor to convert to Christianity, and therefore accepting it in Rome, from which point, Jesus' teachings and the conversion to Christianity had an explosion of growth.