How would you decide this?

Dave Perry’s 100 Best Racing Rules Quizzes highlights specific aspects of the racing rules in a fun format designed to help you become more familiar with The Racing Rules of Sailing. Here is one of the quizzes:

Quiz 61
Boat A is approaching the leeward mark in first place. A nearby race committee boat is making some sound signals and has a flag and two boards displayed. The flag is a blue, white, red, white, blue flag with horizontal bars, a red rectangular board, and a board with a large horizontal sign on it. What do those signals mean? Answer below.

Quiz 61 Answer:
The signals mean the mark ending the next leg has been changed (flag C, which is the blue, white, red, white, blue flag). The mark has been moved left (red board) and shortened in distance (minus sign). See rule 33, Changing the Next Leg of the Course.

Source: US Sailing