Which of the following may transmit encrypted messages?

As a general rule under FCC Part 97, the transmission of encrypted messages is strictly prohibited in amateur radio. The amateur service is designed to be open, transparent, and self-policing, which requires that all communications be readable by anyone listening. However, there is a critical exception made for the control of space stations (satellites). Telecommand signals from a space telecommand station may be encrypted. This is a vital security measure to prevent unauthorized users from hijacking an amateur satellite or accidentally sending harmful commands that could damage the spacecraft. None of the other options, such as terrestrial repeaters or mesh networks, are granted this exception; their control and data links must remain unencrypted.