The correct answer is D) continue voyages unless in danger areas defined in the advisory or supplemental message. When the Naval Control of Shipping (NCS) is established, ships at sea are generally instructed to continue their voyages unless they are directed to avoid specific danger areas that are outlined in the advisory or supplemental messages issued by the NCS authorities. The ships are not required to stop and await further orders, nor are they expected to maintain radio silence or attempt to make their way to the nearest friendly port, unless these specific instructions are provided. The other options are incorrect because they do not align with the standard NCS procedures, which emphasize flexibility and continued operations, with only targeted restrictions on movement as needed to address specific threats or hazards.