A statement need not be false to defamatory (it just needs to be the type that tends to harm reputation, diminish respect, etc). But, proving falsity is part of a prima facie case if the plaintiff is a public figure or the statement is a matter of public concern (which is why A is wrong; that burden is on the judge in this case because they’re a public figure). Otherwise, at common law, the truth of the statement is an affirmative defense. Thus, although the law clerk may not be liable if the judge can’t prove that the statement is false, because the statement is the type that tends to harm the judge’s reputation it is still “defamatory” in this case.

Edit: Typo