Final answer:
The command used to perform a backup in Unix is 'cpio'. It is designed for archiving files and directories. 'zip' can also package and compress files into a single archive, acting as a backup.
Step-by-step explanation:
To perform a backup in Unix, the command that can be used is cpio. While there is no single standard backup command named backup, Unix provides various tools for backing up files. The cpio command stands for copy input/output, and it is commonly used for archiving files and directories as well as performing backups. Another command that can be used for backup purposes is tar, though it's not listed in your options. However, you also mentioned zip, which is a utility for packaging and compressing (archiving) files. While zip is more commonly associated with compression, it can also be utilized to group files into a single archive file, which effectively can serve as a form of backup.