To delete a directory and all of its contents (including read-only files) recursively without prompting for confirmation, the rm command with the -rf flags should be used.
* rm -r: This option tells rm to remove directories and their contents recursively.
* rm -f: This option tells rm to force the removal of files, ignoring nonexistent files and never prompting for confirmation.
Therefore, the correct command is rm -rf old_project. This command will recursively remove the old_project directory and all its contents without any confirmation prompts, regardless of file permissions.
References: Cisco DevNet Associate Certification Guide, Chapter on Linux and Bash Commands.