Which of the following is are external commands ........

Explanation:

Answer : d
All of above

In MS-DOS, there are two ways commands are executed: internally and externally. An internal command is embedded into the command.com file, and an external command is not and requires a separate file to operate.

For example, if your computer does not have the fdisk.exe file and you try using the fdisk command, you will receive a "Bad command or file name" error message. Fdisk is an external command that only works if fdisk.exe, or in some cases, fdisk.com, is present.

However, as long as MS-DOS is running, internal commands, such as the cd command, are available and do not require other files to run. Each of the commands listed on the MS-DOS help page denote if they're external or internal.