Final answer:
The establishment of a common language supports that the leader, Mu’awiyah, created unity.
Step-by-step explanation:
The supporting detail that best supports that the leader, Mu’awiyah, created unity is A) The establishment of a common language.
During Mu’awiyah's reign as the leader of the Caliphate, Arabic was made into the official language of the state, replacing Greek and other languages. This move helped to unify the diverse regions and populations under the rule of the caliphate, as it provided a common means of communication and administration.