The command used to copy a file named temp.doc from drive C: to drive A: is

Answer: Option B

To copy a file named temp.doc from drive C: to drive A: in Disk Operating System (DOS), the correct command is Option B: Copy c:\temp.doc a:. This command specifies the source file with its full path (c:\temp.doc) and the destination drive (a:).

Option A, "Copy temp.doc to a:," is not a valid DOS command syntax.
Option C, "Copy c: a:," does not specify the file name and is not a complete command for copying a specific file.
Option D, "Copy temp a: c:," has incorrect syntax and does not specify the full path of the source file.

Therefore, the correct answer is Option B: Copy c:\temp.doc a:.