Final answer:
The mayor-council system is not one of the three standard forms of county government; the commission system, council-elected executive system, and council-administrator system are.
Step-by-step explanation:
The question asks which form of county government is not one of the three standard forms. The three primary forms of county government in the United States are the commission system, the council-elected executive system, and the council-administrator system. Therefore, the correct answer is C. the mayor-council system, which is not typically a form of county government but is more commonly found in municipal (city) governments.