Disk copy command in DOS is used to

Answer: Option B

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.