Which of the following is not appropriate for transportation of classified documents?

Using unmarked envelopes for the transportation of classified documents is inappropriate because it fails to indicate the sensitivity and classification of the contents. Proper labeling and marking are essential to ensure that anyone handling the documents is aware of their classification level and can take appropriate precautions to protect them. Unmarked envelopes do not provide any indication that the documents within are classified, which increases the risk of unauthorized access or mishandling.

In contrast, options such as using tamper-proof packaging, labeling for visibility, and utilizing secure courier services are all best practices designed to protect classified information during transport. Tamper-proof packaging ensures that the contents cannot be accessed or altered without detection, while clear labeling helps in identification of the documents' classification. Secure courier services are vetted and trained to handle sensitive materials, adding an additional layer of security during the transportation process.