Which is not an operating system?

Correct option is D

Explorer is a file management tool within the Windows OS, not an operating system itself. Operating systems manage computer hardware, software resources, and provide services for computer programs.
Important Key Points:
1. Role of Operating Systems: Operating systems, like Windows, Unix, and Mac OS, control basic functions and resource allocation for applications.
2. Explorer as a Tool: Explorer allows users to navigate and manage files within Windows.
Knowledge Booster:
· Microsoft Windows: A widely-used operating system by Microsoft for desktops and servers.
· Unix: Known for stability and used in server environments; it's an early OS from which many others are derived.
· Mac OS: Operating system designed by Apple, known for its intuitive interface and integration with Apple hardware.