What Is the Purpose Behind Adoption of an Official Language

What is the purpose behind adoption of an official language?

A) To encourage the flourishing of official language groups as a component of the democratic ideal
B) To encourage greater immigration of people from parts of the world where that language is spoken
C) To strive for communication in a limited number of languages in a particular country for greater national unity
D) To appease lobby groups who have been particularly exemplary advocates for their cause.