The correct answer is rsync -a /local/directory user@remote:/backup/directory because the -a (archive) flag is used to preserve permissions, timestamps, ownership, and to ensure recursive copying — while also optimizing by transferring only the differences from the last sync. The --dry-run flag would only simulate the transfer without actually copying files, --update will skip files newer on the receiver without considering other changes, and --in-place could potentially reduce transfer efficiency as it updates destination files in place - useful for large, single files but not necessarily for directory syncs with minimal changes.