DOS stands for Disk Operating System, a computer operating system that uses a storage device like a floppy disk or hard disk. It was the dominant OS for personal computers in the 1980s and early 1990s, with MS-DOS being the most famous version. Option A incorrectly says 'Digital,' Option C incorrectly says 'Dual,' and Option D incorrectly says 'Double.' The name reflects that it managed disk-based file systems and operations.