Explanation: FCC regulations, particularly those governing stations in the maritime services operating in the 405-525 kHz band, stipulate specific identification requirements. When a radiotelegraph station maintains a transmission for more than 20 minutes, it is required to transmit its call sign at least once every 20 minutes during that prolonged transmission. This ensures regular identification for regulatory compliance, station monitoring, and operational clarity. Option B, "At least every 10 minutes," and option C, "At 15 minute intervals," are incorrect as they do not align with the established regulatory interval for this specific service and duration. Option D, "At the beginning and end of each communication," describes general identification practice for discrete exchanges, but does not address the additional requirement for periodic identification during continuous transmissions exceeding 20 minutes.