Disk copy command in DOS is used to

In Disk Operating System (DOS), the command used to copy the entire contents of one floppy disk to another is Diskcopy. This command is specifically designed for duplicating the contents of a floppy disk, including both system and user files, to another floppy disk.

Option A, "Copy a file," refers to the Copy command, which is used to copy individual files, not entire disks.
Option C, "Copy contents of CD-ROM to another," does not apply to Diskcopy, as Diskcopy is specifically for floppy disks, not CD-ROMs.

Therefore, the correct answer is Option B: Copy contents of one floppy disk to another.