Choice B is the best answer. The passage states that historically, "newspapers such as The Times and broadcasters such as the BBC were widely regarded as the trusted shapers of authoritative agendas and conventional wisdom" (lines 27-30,"Historically...wisdom"). But it goes on to say that "there is a growing feeling . . . that the news media should be 'informative rather than authoritative'" (lines 70-73,"There is...authoritative"). Together these lines indicate the main purpose of the passage, which is to discuss how people's perception of the news media is changing from its being an authoritative voice to simply an informative one.
Choice A is incorrect because the passage deals with changes in the way news is perceived but does not primarily focus on the technological changes that may have resulted in those or other changes. Choice C is incorrect because even if the passage implies that viewers might increasingly believe a journalist's values can affect the news stories being produced, it does not provide specific examples of that happening. Choice D is incorrect because the passage begins with the simple sentence "The news is a form of public knowledge" (line 1,"The news...knowledge") and makes no attempt to refute that claim.