A vessel which is being assisted by an icebreaker should indicate that he is ready to cast off the towline (if one is used) by __________.

Correct Answer: Option B — displaying code letter "Y" (Yankee)

International ice-collaboration procedures establish that a vessel being assisted by an icebreaker signals its readiness to cast off the towline by displaying the International Code Flag 'Y' (Yankee) from its halyard.