A system administrator wants to perform a directory synchronization from a local folder to a remote backup server. The requirement is to ensure that only the differences since the last sync are transferred to minimize network usage. The sync needs to be recursive to include all subdirectories and also preserve file permissions. Which command accomplishes this goal most efficiently?
rsync --in-place /local/directory user@remote:/backup/directory
rsync --update /local/directory user@remote:/backup/directory
rsync --dry-run /local/directory user@remote:/backup/directory
rsync -a /local/directory user@remote:/backup/directory