Correct answer is D. A) positive information flows freely through the communication channels of a publicly traded company - This is not an assumption on which the conclusion is based on. How does it have an impact on the conclusion that public has less negative information?
B) employees of a publicly traded company never leak private information about that company to the market - This is a cleverly designed answer with the intent of swaying the test taker to select this. Private information does not necessarily mean negative information.
C) CEOs and other members of the most private circles of a publicly traded company are willing to distort the perceived truth about the company in order to defend the company's share price - Incorrect answer. This does not help hold the conclusion. Remember that an assumption should always help the conclusion to be drawn without any question.
D) the public obtains negative information about a public company only from sources within that company - Correct. Only if the author considers that public cannot obtain negative information about the company from other sources, can the author provide the said conclusion. The argument fails if the author does not consider this premise as true.
E) the public wants all relevant information about a publicly traded company, positive and negative - This is irrelevant for the conclusion. Remember that an assumption is a necessary component of an argument