In which of the following operating systems can HTML documents be viewed?

HTML documents can be viewed in all mentioned operating systems: Mac OS, Linux, Windows, and Unix. This capability is primarily due to the wide availability of web browsers such as Safari, Chrome, Firefox, and others that are compatible with these operating systems.

Regardless of the underlying OS, as long as there is a suitable web browser installed, users can open and render HTML files, allowing them to view web content seamlessly. The universality of HTML and the supporting infrastructure of web browsers contribute to this functionality across different systems.

Each of the operating systems listed has multiple web browsers available, further enhancing their ability to handle HTML documents efficiently. This makes option D, which includes all four operating systems, the most comprehensive and accurate choice.