Which command is used to make a new directory?

Answer: Option A

In Disk Operating System (DOS), the command used to make a new directory (folder) is Option A: Md. When you type "md dirname" at the command prompt in DOS, it creates a new directory with the name "dirname" in the current location.

Option B, "Cd," is used to change the current directory, not to create a new one.
Option C, "Rd," is used to remove (delete) a directory, not to create one.

Therefore, the correct answer is Option A: Md.