The phrase in your title — “which method may be used to transmit confid…” — matches a common U.S. Department of Defense (DoD)–style test question:

“Which method may be used to transmit Confidential materials to DoD agencies?”

For Confidential (the lowest U.S. classification level above Unclassified), standard training materials explain that it may be sent by:

  • U.S. Postal Service (USPS) first‑class mail within the United States and its territories, addressed appropriately and packaged so the classification is not visible on the outer envelope.

In contrast, Top Secret and Secret usually require more restrictive means (special couriers, secure communications networks, or registered/express services with receipts and positive control), so simple first‑class mail is not acceptable for those higher levels.

So if your truncated exam/quiz item is the common DoD question, the intended correct method for transmitting Confidential material is:

  • USPS first‑class mail (properly packaged and addressed to the DoD agency).

Information gathered from public forums or data available on the internet and portrayed here.