Final answer:
Muawiya I was the leader who declared Arabic the official language of the government, aiding in the unification of the Umayyad empire. This action was part of a broader effort to Arabize and solidify the identity of the Islamic empire.
Step-by-step explanation:
The individual who helped unify the Umayyad empire by declaring Arabic the official language of the government was D) Muawiya I. As the founder of the Umayyad dynasty, Muawiya I was a shrewd politician who moved the capital from Medina to Damascus and started the Arabization of the government, which included making Arabic the official administrative language. This decision replaced the use of Greek and a variety of other languages, marking a significant step in the unification and identity formation of the Islamic empire.
Regarding the unity of Muhammad's ummah, the option that best fits based on historical data is b) the belief in one God, Muhammad's leadership, and mutual defense. The use of the Arabic language helped solidify this unity under the Umayyads later on, but it was not the initial factor that united Muhammad's followers.