The correct answer is D) Jerusalem.
The area that practiced Christianity in 600 CE was Jerusalem.
People practiced Christianity in Jerusalem at that time, but in a matter of years, it expanded to new territories throughout the Roman Empire.
At the beginning of Christianity, it was spread by the disciples of Jesus and by Paul of Tarsus who was converted by a vision of Christ. From there, he became a promoter of Christianity. By the third century C.E., Christianity was the dominant religion in the Mediterranean region. After that, the Roman Empire expanded to the north and west of the continent, spreading the doctrine all across Europe.