The historian is saying that there is no direct evidence of timber trade, but a law setting tariffs on timber suggests that a timber trade was conducted. The critic responds by drawing an analogy to modern statutes that concern activities that no one engages in.
(C) is incorrect because the historian says that the statutory evidence suggests that the trade was conducted, "suggests" implies a possibility rather than certainty. Hence, the critic is not saying that the reasoning is flawed because it ascertains something that is a mere possibility.
