The correct answer is C. Islamic caliphates spread Islam throughout the region during the postclassical period.
In Historical studies, the post-classical period is the name for the period after the fall of the Roman Empire at the end of the 5th century AD. Islam emerged as a major religion in the Arabian peninsula at the beginning of the 7th century and it rapidly spread throughout the Middle East, North Africa, and beyond. The reason why it extended so fast was that Islam was organized in a central Islamic empire called Caliphate, which was in charge of spreading the religion through preachers and mainly through the conquest of new territories.