Explanation: The band 480 to 515 kHz encompasses 500 kHz, which was historically the international radiotelegraph distress frequency for maritime use. To ensure that weak distress calls could be heard, international regulations mandated specific "silent periods" during which routine transmissions were forbidden. These periods were typically three minutes past the hour and three minutes past the half-hour, when all stations were required to maintain silence and listen for distress signals. Therefore, option C is correct because it identifies this specific regulatory requirement designed to prioritize distress communications. Option A is incorrect because while the band is heavily regulated and not generally available for routine amateur use, transmissions for distress or safety purposes are certainly allowed, and routine transmissions might be permitted at other times by authorized stations. The question asks *when* they are forbidden, implying a specific timeframe. Option B is incorrect. The 480-515 kHz range is, in fact, historically central to marine radio, especially for distress communications. Option D is incorrect because C provides the correct explanation.