The correct answer is C) restricted radiotelephone operator permit. The reason is that the regulations require the operator of a ship's radiotelephone, if carried voluntarily, to hold at least a restricted radiotelephone operator permit. This permit demonstrates the operator has the necessary knowledge and skills to properly operate the radiotelephone equipment onboard. A mate's license, seaman's document, or second-class radio operator's license are not the minimum required qualifications for this specific duty.