Which of the following is NOT an operating system?

HTML, or HyperText Markup Language, is a standard markup language used for creating and designing documents on the World Wide Web. It is used to structure content on web pages by defining elements such as headings, paragraphs, links, and other components of a website. Unlike operating systems, which manage computer hardware and software resources and provide services for computer programs, HTML does not perform these functions. It simply dictates how content should be organized and displayed in web browsers.

On the other hand, Windows, macOS, and Linux are all operating systems. They serve as the backbone of computer systems, managing resources, running applications, and providing an interface for user interaction. These operating systems are essential for the functioning of a computer, while HTML plays a supportive role in web development, without including the core functionalities of an operating system.