Which of the following is a PC operating system?

The correct answer is DOS. DOS, which stands for Disk Operating System, is a command-line based operating system used primarily in personal computers. It was among the earliest operating systems that allowed users to manage files and run software on IBM-compatible PCs. Its simplicity and direct interaction with the hardware made it a foundational OS for early computing on personal computers.

While other operating systems like Android, Ubuntu, and Mac OS have their own functionalities and user environments, they do not all qualify as traditional "PC operating systems" in the same sense as DOS. Android is primarily designed for mobile devices, Ubuntu is often categorized under Linux-based systems and might not traditionally be referred to simply as a "PC operating system" without context, and Mac OS is tailored specifically for Apple hardware. Each of these systems has its own distinct use cases and environments that set them apart from DOS in the traditional realm of PCs.