Definitely D. The wording of the question is misleading, as many of them are. It is not true that one of the answers weakens the argument more than another or some others; one of the answers DOES weaken the argument, and the others DON'T.
The conclusion claims that the "skill, care and effort" of carpenters who worked on hotels before 1930 exceeded that of carpenters who worked on hotels after 1930. This is an extremely specific conclusion, and it does not have any of the more common loopholes -- such as comparing carpenters who worked on hotels before 1930 to ALL carpenters afterwards. The evidence is also quite specific: The author has seen that the "quality of the original carpentry work" (which clearly excludes materials, artistry of design, etc.) is generally better in hotels that were built before 1930 than in hotels that were built afterwards.
I for one was not able to see a missing assumption in this argument; it looks quite solid. But it is a Weaken question, and that means that there must BE a missing assumption. One of the answers must undermine or contradict that assumption, because that is how an argument is weakened on the GMAT. So let's look at the answer choices, and see which one points at the assumption that escaped our notice.
(A) Irrelevant. The evidence compares carpentry work in two groups of hotels, and the author reaches a conclusion about carpentry work in those two groups of hotels. How well the older carpenters did their work OUTSIDE of hotels does not matter.
(B) Irrelevant. This might be seen as a reason why carpenters after 1930 were less careful, but that does not affect the argument. The argument STARTS from the observation that the quality of the work in the newer hotels is not as good as in the older hotels, and then reaches a conclusion about the carpenters' behaviour. In order to weaken the argument, we need to show that OBSERVING lower quality does not necessarily mean that the carpenters WERE less careful. Why they may have been less careful does not affect the linkage between the observation and the behavioural conclusion.
(C) Strengthens the conclusion. It eliminates the possibility that the quality in the newer hotels is worse because the materials were actually so bad that even with the same level of care on the carpenter's part, the result looks bad. (Notice, however, that we have identified at least one assumption here. The argument assumes that the materials after 1930 were not so bad that they affect the visible quality of the work. Unfortunately, the answer choice supports the assumption rather than contradicting it.)
(D) This one says that a building with lower quality carpentry is more likely to be destroyed. Obviously, hotels built before 1930 have had more time to be destroyed than hotels built afterwards. THIS weakens the argument; it says that the guidebook writer is much less likely to SEE the hotels which were built with bad carpentry before 1930, because those are most likely to have disappeared. Consequently, the hotels STILL STANDING which were built before 1930 may not be a representative sample of all hotels which were built back then. Now we can see the assumption, which is that the hotels seen by the writer are representative of both the pre-1930 and the post-1930 time periods. (D) undermines that assumption, and weakens the argument.
(E) This is somewhat like (B). It could explain why later carpenters did not HAVE as much skill as earlier carpenters. It is irrelevant to the argument, however, which starts from observational evidence and concludes that they either did not have as much skill or did not use as much skill/care. To weaken the argument, we have to attack the linkage between the observations and the conclusion, and only (D) does that.
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Grumpy
Kaplan Canada LSAT/GMAT/GRE teacher and tutor
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