Which document must accompany classified information during transit?

The correct answer is the classified information transmittal document because this specific document serves a critical function in the handling and transport of classified information. It provides essential details that ensure the information is properly tracked and accounted for, including the classification level, the name of the sender and recipient, and the handling instructions. This documentation is vital in maintaining the security and integrity of classified materials during transit.

In contrast, a notarized letter does not fulfill the specific needs related to classified information transfer, as it is generally meant for authentication purposes rather than security compliance. A declassification notice is not applicable for classified materials in transit, as it indicates a change in the status of information rather than the protocols required for movement. Lastly, a personal identification form is related to individual identities rather than the transit of classified information itself and does not provide the necessary details for the secure handling of such materials.