4th rule of this game

The contrapositive of the fourth rule is “If she chooses N, then she cannot choose L or M at the previous stop.” (D) doesn’t have L or M preceding either of the Ns, so it passes that rule. (As an aside, you don’t need to apply the contrapositive to the answer choices when doing an Elimation question like this one. To apply the fourth rule, I’d only check to make sure there aren’t any Ns following any L or Ms in the answer choices)