Oh, Analogy questions, you are my least favorite.
I think you are missing a couple tidbits from the passage that make (A) a stronger match.
In the question stem, they want an analogy for the relationship between "pre-20th century" building methods and "new methods" of building construction".
Where do we hear about "pre-20th century" vs. "new" building methods?
Lines 40-44 tell us that "pre-20th" = a small number of workers did the job. "New" = many specialized subcontractors work independently.
Lines 44-53 go on to describe that the pre-20th century method allowed for more consistency (fewer people doing the job) and a higher level of craftsmanship. By contrast, the new method of having many different subcontractors each perform a specialized task now necessitates that an architect expect "an allowable degree of inaccuracy of the different trades".
So the contrast between "assembly line" and "single garment worker" is supposed to match up with "NEW = many specialized workers" and "pre-20th = relatively small number of workers".
Clearly, it's not an ideal match, since there was not a "single builder" in pre-20th century building. But the idea that assembly line production leads to a lower quality of craftsmanship is definitely reinforced by lines 40-53.
Why would LSAT expect that we would have remembered this point/moment in the passage? This forms a large part of the author's criticism of the Modern Movement. Ultimately, the author thinks that the Modernist's desire to have exposed structural elements (line 54) would be unrealistically expensive, given that buildings are now made 'by committee'.
Hope this helps.