So, the historian is saying that though there is no direct evidence that timber was traded between the ancient nations of Poran and Nayal, the fact that a law setting tariffs on timber imports from Poran was ENACTED during the third Nayalese dynasty suggests that the timber trade was--in fact--conducted during that period.
The Critic tries to argue that the historian's reasoning is flawed because on today's statute books there REMAIN many laws that regulate activities that were once common, but in which people no longer engage.
There is a disconnect between the historian using evidence that a law setting tariffs on timber imports from Poran was ENACTED and a law in the statute books today about regulating activities is REMAINING.
The historian is saying that trade probably was conducted since there would have been no need to enact a law to conduct trade if no trade was being conducted. Whereas, the critic is saying that since there are many laws today that cite to laws regulating activities that were once common--in this case, let's say trade being conducted--despite the fact that people no longer engage in them, the historian's evidence is not sufficient to conclude that Nayal imported timber from Poran.
SO, essentially, the critic is explaining that just because there REMAIN laws in a statute book that cite to trade being conducted, that does not actually mean that the trade is currently being conducted.
However, the critic is ignoring the point at issue, which is that the historian is saying that facts show that a law setting tariffs on timber imports was ENACTED, not found to still be REMAINING. A law being ENACTED is much different from a law that REMAINS in the statute books and may or may not have become obsolete.
Therefore, the critic's evidence does not affect the historian's reasoning; the critic is ignoring the point at issue: that a law setting tariffs on timber imports from Poran was actually ENACTED during the third Nayelese dynasty.
That is exactly what answer choice (D) is pointing out: "takes no account of the difference between a law's enactment at a particular time and a law's existence as a part of a legal code at a particular time."
Hope that clears things up! Please let us know if you have any other questions.
