To copy the file command.com from drive c: to drive a:

Answer: Option C

In Disk Operating System (DOS), to copy the file command.com from drive C: to drive A:, the correct command syntax is essential.

Option A: Drive c: copy drive a:\command.com. This option is incorrect because it does not follow the correct syntax for the copy command in DOS.

Option B: C:A: copy command.com. This option is incorrect because it does not specify the source and destination paths correctly.

Option C: Copy c:\command.com a:. This option is correct as it specifies the source file c:\command.com and the destination drive a: using the copy command in DOS.

Option D: Both b and c. This option is incorrect because Option B is incorrect. Option C alone provides the correct syntax.

Therefore, the correct answer is Option C: Copy c:\command.com a:. This command will copy the file command.com from drive C: to drive A: in DOS.