Under which of the following circumstances are one-way transmissions by an amateur station prohibited?

One-way transmissions by an amateur station are prohibited notably in the context of broadcasting. The essence of amateur radio is to encourage two-way communication and experimentation among licensees. Broadcasting, defined as the transmission intended to be received by the general public and not restricted to amateur operators, falls outside the regulatory scope of amateur privileges. Therefore, engaging in one-way transmission for broadcasting purposes contradicts the fundamental principles and regulations governing amateur radio operations.

The other contexts mentioned, such as international Morse code practice, telecommand, or telemetry, can involve one-way transmissions under certain circumstances that comply with amateur radio rules. For instance, sending telemetry data is allowed because it relates to the monitoring of space-related operations and can serve educational or experimental purposes. Thus, the regulation against broadcasting effectively highlights the distinction between amateur and commercial or public communication.