Which one of the following writers observed that “Man’s ignorance of law cannot be excused because he…

The observation is Blackstone's: in his Commentaries he wrote that a mistake in point of law, which every person of discretion is bound and presumed to know, is no defence. Austin did not accept that reasoning; in his Lectures on Jurisprudence he rested the rule instead on the practical difficulty courts would face in deciding whether a person really was ignorant of the law. Bailey and Kelsen are not linked with the statement, so none of the writers listed is right.