How long must a radio log be kept on board before being sent to the shoreside licensee?

The requirement for keeping a radio log on board for at least 30 days after the last entry is based on regulations that govern the documentation and maintenance of maritime communications. This specific duration allows for sufficient time to ensure that any significant communications or operational details can be reviewed or referenced if necessary after the last operation has been documented.

Retention of the log for this period serves several purposes, including compliance with regulatory standards, enabling audits, and ensuring that any communication can be traced if issues arise. After 30 days, the information in the log becomes less critical for ongoing operations, thereby allowing the licensee to transmit these records to shoreside for further retention or compliance needs.

The other durations listed in the choices exceed the requirement and do not accurately reflect the established regulations for radio log retention on board vessels, which is why they are not the correct answer.