A vessel which is being assisted by an icebreaker should indicate that she is ready to cast off the towline by __________.

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

Standard international icebreaker signals require the assisted vessel to display the International Code Flag "Y" (Yankee) during daylight or sound corresponding whistle signals to indicate readiness to cast off the towline upon successful transit through heavy ice.