From lines 38 to 44, the author tells us that the building industry became "more specialized and the process of construction was much more fragmented than in the past. Up until the twentieth century, construction had been carried out by a relatively small number of tradespeople, but as the building industry evolved, buildings came to be built by many specialized subcontractors working independently." This led to a "degree of inaccuracy" (line 47) which meant that Modernism now "called for an unrealistically high level of craftsmanship" (lines 52 to 53).
In essence, the difference was that there were fewer tradespeople who used to have more generalized skills the twentieth century when buildings were constructed by many independent specialized subcontractors. This fragmentation of the construction process led to a decrease in accuracy.
Answer choices (B) and (E) are incorrect because the change was not aesthetic.
Answer choice (C) is incorrect because, as far as we can tell, the change did not affect the utility of the buildings.
Answer choice (D) is incorrect because we learn nothing about what happened after these new buildings were built.
This leaves us with answer choice (A). Notice that this analogy directly mirrors what happened with the construction industry.
Clothing produced by an assembly line is less precisely tailored than clothing produced by a single garment maker.
Buildings constructed by specialized subcontractors are less precisely constructed than buildings constructed by fewer, more generalized tradespeople.
Hope this helps!
Please let us know if you have any further questions.