Final answer:
The correct method to transmit confidential material to a military organization overseas is through secure military channels, which provide the necessary security over standard postal services, private courier services, or fax transmissions. The correct option is C.
Step-by-step explanation:
To transmit confidential material to a military organization located overseas with an APO/FPO address, the proper method would be through secure military channels. Using standard postal service or private courier services may not provide the necessary level of security and encryption required for confidential military communications.
Fax transmission also does not ensure the level of security needed for such confidential material. Therefore, secure military channels are the preferred and appropriate method for such transmissions to safeguard the information. The correct option is C.