Which option is permitted as the first line in a simplified address?

The first line in a simplified address must be a term that is broad enough to encompass various recipients without specifying an individual's name. "Postal Customer" fits this criterion perfectly as it is a general term that can apply to any individual receiving mail at that address. This term effectively indicates that the mail is intended for any person who resides at or is designated to receive mail at the specified address.

The other options, while they may seem similar, do not provide the same level of clarity or acceptability within the context of simplified addressing. "Dear Postal Customer" adds unnecessary formality and specificity that goes beyond just addressing the recipient. "Mail Recipient" and "Residential Recipient" also lack the generality and simplicity required for the first line of a simplified address.