The rules that decide who turns. Two rulebooks and where the line between them is drawn, the look-out and safe-speed duties, how you know a collision is coming, what a proper avoiding action looks like, the three encounters, the two stages of the stand-on vessel’s duty, and the hierarchy in Rule 18 with every exception that eats into it.
Score 80% or higher to earn a certificate.
Where the International and Inland Rules differ, the question says which one it is asking about. A question that does not say so has two correct answers, and that is the single commonest fault in boating quiz material.