Which operating system is characterized as open-source with freely available source code?

The choice of Linux as the correct answer is appropriate because it is a widely recognized open-source operating system that allows users to access, modify, and distribute its source code freely. This open-source nature promotes collaboration and innovation within the community, leading to a variety of distributions and adaptations that cater to different needs.

In contrast, while Mac OS X and Chrome are popular operating systems, they are not open-source. Mac OS X, developed by Apple, has a closed-source model where users cannot freely modify its underlying code. Similarly, Chrome, which is primarily a web browser with some elements derived from the open-source Chromium project, does not represent a fully open-source operating system.

UNIX, while foundational in the development of many operating systems, including Linux, is not considered open-source in the same context as Linux. Traditional UNIX systems often have proprietary licenses, limiting the access and modification of their source code. Thus, Linux stands out distinctly as the open-source operating system with freely available source code.