Yeah, I totally hear you.
For those of us who know we should prove our answers with line references, the correspondence between (A) and 42-43 is super weak.
However, the question stem does not say "what can be inferred" but "what is most supported". In both RC and LR, it's important to realize that the latter phrase potentially allows for more leeway in stretching what you've been told.
And in modern RC sections, I've found that there are a few questions per section in which the correct answer is definitely better than the others but not as "provable" from the text as what I'm used to.
We really have to choose this stinker based on the others being even farther removed from what we're told in lines 42-43 (and trusting that those lines really are our only window of text to consider).
(B) is close to the previous sentence, which says that Porter's goals in painting better attuned him to certain subtleties in his research. But "fueled his interest in art history" is quite a leap.
(C) there's no evidence Porter's paintings appeared in his book
(D) "deliberate" and "prove" make this very extreme (on top of being unfounded)
(E) "after ALL" his work had been completed makes this very extreme (on top of being unfounded).
So it's definitely either stretch 42-43 to pick (A) or stretch the previous sentence to pick (B). Ultimately, it seems like (A) is the safer idea, the smaller stretch.
And welcome to the world of hating the correct answer.
Good luck.