The term political subdivision includes any subdivision of a public unit, any principal department of such public unit:

(a) The creation of which subdivision or department has been expressly authorized by state statute,

(b) To which some functions of government have been delegated by state statute, and

(c) To which funds have been allocated by statute or ordinance for its exclusive use and control. It also includes drainage, irrigation, navigation, improvement, levee, sanitary, school or power districts and bridge or port authorities and other special districts created by state statute or compacts between the states. Excluded from the term are subordinate or nonautonomous divisions, agencies or boards within principal departments.