Choice D is the best answer. In lines 11-12("There...decision"), the author introduces the main purpose of the passage, which is to examine the "different views on where ethics should apply when someone makes an economic decision." The passage examines what historical figures Adam Smith, Aristotle, and John Stuart Mill believed about the relationship between ethics and economics.
Choices A, B, and C are incorrect because they identify certain points addressed in the passage (cost-benefit analysis, ethical economic behavior, and the role of the free market), but do not describe the passage's main purpose.