Which tar option prevents the archive from spanning across different file systems?

The option that prevents the tar archive from spanning across different file systems is the -l flag. When this option is used, tar will stay within the confines of a single file system and will not include files from mounted other file systems in the archive. This can be particularly useful when you want to ensure that the contents of the archive are limited to a specific directory or volume, avoiding unintended inclusions from directories that might be linked to other file systems.

The other options do not serve this purpose. The -f option is used to specify the filename of the archive, -g is utilized for incremental backups, and -z is for compressing the archive using gzip. Hence, -l is specifically designed to limit the scope of the archive to the current file system, making it the correct choice for this scenario.