The correct answer is C: Only when transmitting control commands to space stations or radio control craft. According to FCC Part 97.113(a)(4), messages encoded to obscure their meaning are permitted only for telecommand operations - controlling space stations or radio control craft (like model aircraft, boats, or vehicles). This exception exists because telecommand signals for controlling remote devices often use encoded formats that may appear to obscure meaning, but they're actually control protocols, not encrypted communications. All other amateur communications must be in plain language that can be understood by any listener. This ensures that amateur radio remains an open, transparent service where communications can be monitored for compliance with rules. Encrypted communications for privacy are not permitted, except for this specific telecommand exception.