The answer is - B. Mass migrations from a troubled region.
Mass migrations are often a reason for cultural divergence. Because a large number of people suddenly come to a place where the cultural traits are much different, they tend to stick together and form close communities where they keep their cultural traits and way of life mostly untacked by the new surrounding (example for this are the new waves of Muslim migrants to the west). This also contributes that the population that is native to that region acts more defensively towards its own cultural traits so they also form closer communities where they try to preserve their culture untacked by any foreign or unwanted influence (example for this are the Amish people in the USA).
