Answer :
I believe the answer is: A.His empire became too large for him to control.
At the peak of its power, Sargon of Akkad managed to control almost all area in Mesopotamia.
Since the area of his conquest became really large, it is borderlien for him to govern all area in a centralized system like smaller empire does, so he let his territory into several city states and gave them the a certain level of autonomy to govern themselves.