There are two parts to the conclusion: /1/ the artist will make more profit per painting, and, therefore, /2/ her total income will go up as well.
Our reasoning here should focus primarily on part 2 of this conclusion, since that's the final, main conclusion of the whole passage—but, technically, the correct answer could be something that weakens part 1, part 2, or both.
Weakening part 1 means pointing to a reason why the artist's
profit per painting might be the same as, or even less than, before.
Weakening part 2 means pointing to a reason why the artist WON'T make more money overall, even if she sells each painting at a greater unit profit. A higher unit profit could only coincide with lower total revenue if the artist sells fewer total units—so a weakener of part 2 would be any reason
why the artist will sell fewer paintings after making the changes described here.
.
Looking at the choices in terms of the criteria above:
(A) Irrelevant. We don't care about any other artists; the passage focuses exclusively on this one artist.
(B) The argument is limited to what will happen if the artist makes the specified changes. Things she DIDN'T do are irrelevant.
(C) This choice supplies a solid reason why the artist will sell fewer paintings after making the changes—so this is a weakener of part 2 above and thus the
CORRECT ANSWER.
(D) There's no reason to think that a protracted period of inflation is likely to happen, so there's no reason to take that scenario into account.
(E) Same issue as B: We don't care about alternatives that the artist decided NOT to pursue, regardless of her reasons for forgoing them.