I would go with D.
D because, only the best quality buildings built before 1930 remained. And all the buildings that were built without that level of skill, care, effort were demolished. So whatever buildings that remain from before 1930 are naturally better in terms of quality compared to those built later.
Argument says "noticed that in those built before 1930 the quality of the original carpentry work is generally superior
to that in hotels built afterward"
Generally superior and not true in all the cases. Generally superior because the sample is not representative of the all the buildings built before 1930 and after 1930. D is weakening the argument.
E says: The average length of apprenticeship for carpenters has declined significantly since 1930. A possible consequence is less mastery, and less skill in the work done. E is not weakening, and to a certain extent strengthening the argument.
The path is long, but self-surrender makes it short;
the way is difficult, but perfect trust makes it easy.