This question asks you for a main purpose of a specific paragraph. It is important to be aware of the structure of the passage for this type of question.
Third paragraph is mainly about the distinction of what is NOT abuse of monopoly power nor a violation of antitrust law associated with supra competitive power. It intentionally starts with the possible association between supra competitive power and consumer's welfare to get into why it's NOT always associated with abuse of monopoly power. And then it goes on to support the claim with the example of "economies of scale", then summarizing the paragraph with why it's not a violation of antitrust law.
Now, let's go to the answer choices.
A) Although we could speculate what it is, supra competitive "profits" are not covered in the paragraph.
B) This paragraph is describing what is NOT an abuse of monopoly of power, and not being an abuse doesn't necessarily indicate being positive.
C) This is used as an example to show why possessing and using monopoly power itself doesn't indicate the abuse. It’s not the main purpose.
D) Just as discussed above, the third paragraph distinguishes what is not covered by the antitrust laws under discussion (prohibiting monopoly power as a whole + using economies of scale) from what is covered (abuse of monopoly power). This is it. (Correct)
E) As far as we know, a term "customer welfare" is used for the first time in this paragraph. No reminding.