An icebreaker may use the code letter "K" to remind ships of their obligation to listen continuously on their radio. This signal may be made by any of the following EXCEPT __________.

Correct Answer: Option C — radiotelephone

International ice navigation signals permit the letter 'K' to be transmitted visually via light signal, sound signal, or visual flag hoist to remind vessels of continuous radio guard obligations. Radiotelephone voice transmission is excluded since the signal itself prompts radio usage rather than being broadcast over it.