Which document serves as proof of permanent residency in the United States?

The document that serves as proof of permanent residency in the United States is the I-551. Also known as the "Permanent Resident Card" or Green Card, the I-551 is issued by U.S. Citizenship and Immigration Services (USCIS) to non-citizens who have been granted authorization to live and work in the U.S. on a permanent basis. This card contains the holder's personal information and expiration date and is a critical form of identification for permanent residents, as it attests to their lawful status in the United States.

Other options, such as the I-766, which refers to Employment Authorization Document, provide proof of the right to work but do not confirm permanent residency. The I-94 is primarily used to record the arrival and departure of non-citizens and does not imply permanent residency status. The I-840 is also not relevant as it does not pertain to residency status. Therefore, the I-551 is the only document listed that conclusively proves permanent residency within the United States.