The correct answer is:

(A) DOS

DOS (Disk Operating System) is a CUI (Character User Interface) operating system. It relies on text-based commands, and users interact with it through a command-line interface rather than graphical windows.

The other options (Windows 10, Windows 7, and Windows XP) are GUI (Graphical User Interface) operating systems, which use graphical elements like icons, windows, and buttons for user interaction.