Which of the following is NOT a common operating system?

The rationale for identifying HTML as the choice that is not a common operating system lies in understanding what constitutes an operating system versus other types of software. Operating systems are system software that manage computer hardware and software resources and provide common services for computer programs.

Microsoft Windows, Apple Mac OS, and Android are all examples of operating systems. They provide the necessary environment for applications to run, handle hardware interactions, and offer user interfaces for interaction with the hardware.

On the other hand, HTML (HyperText Markup Language) is a markup language used for creating and structuring content on the web. It is essential for web development, but it does not perform the functions of an operating system. Instead, it works within the frameworks established by operating systems to render web pages. Therefore, it is correct to conclude that HTML is not an operating system, distinguishing it from the other options provided.