Explanation: Radio logs for ship stations, as governed by FCC Part 80 regulations for the maritime services, must be retained on board the vessel for a period of at least 30 days from the date of the last entry. This rule, specified in 47 CFR §80.409(e)(2), ensures that recent operational records are readily available for inspection by authorized personnel, such as FCC or Coast Guard officials, while the vessel is in service. Options B, C, and D are incorrect because they state longer retention periods that do not align with the specific 30-day requirement for keeping the log *on board* the vessel before it is typically transferred to the shoreside licensee for long-term archival. While the licensee may have a longer overall retention requirement, the initial on-board period before transfer is 30 days.