The correct answer is D) the radio room, bridge, or protected location. The required portable radio apparatus on an international voyage must be stowed in a protected location, such as the radio room, bridge, or another secure area. This is to ensure the radio equipment is easily accessible and protected from damage in the event of an emergency. The radio apparatus is a critical piece of safety equipment on international voyages, so it must be stored in a location that is both readily available and secure. The other answer choices are incorrect because they do not meet the regulatory requirements for the storage of the portable radio apparatus. Storing the radio in the ship's office, the Master's quarters, or an unlocked cabinet would not provide the necessary protection and accessibility in an emergency situation.