It is not a difficult one to answer.
if you look at the second part of the passage
Quote:
But trends following the Civil War produced a drastic shift away from the adventurous optimism of the pre-war era and toward a more subdued appreciation for the details of American life. In this new social context, the paintings now seemed too decadent, too gaudy, for the new philosophy taking root in the country following the horrors of war. As one commentator in 1866 put it, Bierstadt’s work “may impose upon the senses, but does not affect the heart.” In a sense, then, that same American pride upon which Bierstadt had capitalized to advance his success was now, in its fickleness, the source of his downfall.
The sense is that after the civil war the American mood was somehow less blatant and quieter, enjoying more the daily life and small things that are priceless.
His paintings at this point are a bit out of scope because they are too gaudy or flashy.
The quote is to explain just this concept: what before was his fortune now is his own decline.
E is the answer.
Hope now is clear.