Which of these situations is an example of a diaspora?

A. A group of people from one country establishes a colony in another country, and many people decide to move to the colony.
B. A family of four decides to migrate to another country, but they are unable to make a living there and eventually move back home.
C. Two countries agree to exchange half their populations in order to increase cultural diversity in certain communities.
D. Many people from a country are forced out by a conquering nation and settle in communities scattered throughout the world.