The following command set is correct according to their function

Answer: Option C

In Disk Operating System (DOS), the command set that is correct according to their function is Option C: CD and RD.

CD (Change Directory) is used to change the current directory in DOS.
RD (Remove Directory) is used to delete or remove a directory in DOS.

Option A, "RD can MD," is not a correct command set. MD (Make Directory) and RD (Remove Directory) are commands for creating and deleting directories respectively, but "RD can MD" is not a valid syntax.
Option B, "DEL and ERASE," are commands used for deleting files, not for creating or managing directories.
Option D, "COPY and RENAME," are commands used for copying files and renaming files, not for managing directories.

Therefore, the correct answer is Option C: CD and RD.