Final answer:
The example of a network operating system out of the given choices is Windows 7. While the other options are types of networks, Windows 7 is an operating system that supports networking functions.
Step-by-step explanation:
The question 'Which of the following is an example of a network operating system?' refers to software that manages network resources and allows communication between multiple devices on a network. Out of the options provided: the internet, extranet, WAN (wide area network), and Windows 7, only Windows 7 can be classified as a network operating system. Internet, extranet, and WAN are types of networks or methods of organizing and sharing information, not operating systems. Windows 7, on the other hand, is an example of an operating system that can manage network resources and has the capabilities to support networking functions like file sharing, printer sharing, and user account management.