Explanation: Ship radiotelegraph logs are essential records of watchkeeping for maritime safety. During a radio watch, entries must be made at least every 15 minutes to confirm the watch is being maintained, demonstrating continuous monitoring of the distress frequencies. Furthermore, specific entries are required for the mandatory silent periods (e.g., 15-18 minutes and 45-48 minutes past each hour on 500 kHz). These entries, confirming that the watch was kept and no distress calls were heard during these critical listening times, are *in addition* to the general 15-minute periodic entries. Therefore, both the 15-minute frequency (A) and the additional nature of silent period entries (B) are correct requirements, making option D the most complete answer. Option C is incorrect because the required general entry interval is 15 minutes, not 30.