The correct answer is A. DOS.
DOS, or Disk Operating System, is a single-tasking operating system. This means that it can only run one program at a time. Other operating systems, such as Linux, Windows, and Unix, are multi-tasking operating systems, which means that they can run multiple programs at the same time.
DOS was developed by Microsoft in the early 1980s and was the most popular operating system for personal computers until the early 1990s. It was a simple and easy-to-use operating system, but it was also limited in its capabilities. DOS was eventually replaced by Windows, which is a more powerful and versatile operating system.
Linux is a Unix-like operating system that was developed in the early 1990s. It is a free and open-source operating system, which means that it is available to anyone to use and modify. Linux is a popular operating system for servers and embedded devices, and it is also gaining popularity as a desktop operating system.
Windows is a family of operating systems developed by Microsoft. The first version of Windows was released in 1985, and it has been the most popular operating system for personal computers since the early 1990s. Windows is a multi-tasking operating system that can run multiple programs at the same time. It is also a graphical operating system, which means that users interact with it using a mouse and a graphical user interface (GUI).
Unix is a family of operating systems that was developed in the early 1970s. It is a multi-tasking, multi-user operating system that is designed to be reliable and secure. Unix is a popular operating system for servers and supercomputers, and it is also used by some desktop users.