Final answer:
The mayor-council system is not a form of county government; it is more commonly associated with municipal government structures and is not one of the typical forms of county government.
Step-by-step explanation:
The question relates to identifying which option is not a form of county government. Among the options given, A. the commission system, B.
the council-elected executive system, and D. the council-administrator system are indeed forms of county governance. However, C. the mayor-council system is typically associated with municipal government rather than county government and is not one of the three common forms of county government structures.