Understanding the argument -
The format of network television news programs generally allows advocates of a point of view only 30 seconds to convey their message. - Background info.
Consequently, regular watchers become accustomed to thinking of issues in terms only of slogans and catch phrases, and so the expectation of careful discussion of public issues gradually disappears from their awareness. - "consequently" introduces a conclusion or effect.
The format of newspaper stories, on the other hand, leads readers to pursue details of stories headed by the most important facts and so has the opposite effect on regular readers—that of maintaining the expectation of careful discussion of public issues. - Opposite effect.
Therefore, in contrast to regular newspaper reading, regular watching of network television news programs increases the tendency to think of public issues in oversimplified terms. - conclusion. Simply put, TVs promote the oversimplified tendency toward public issues. Why? Maybe because they don't "lead readers to pursue details of stories headed."
Option Elimination - Assumption
(A) Viewers of network television news programs would be interested in seeing advocates of opposing views present their positions at length. - but even then, that'll promote oversimplification as the opposing views will also be limited to 30 seconds. It doesn't cover the possibility for TVs to cover the details. Distortion.
(B) Since it is not possible to present striking images that would symbolize events for viewers, and since images hold sway over words in television, television must oversimplify. - Misses out on the detail part. Moreover, what is an assumption? It's a missing premise. What is "must"? Recommendation. So, while not a sufficient condition, the majority of times, such options are not the right candidate.
(C) It is not possible for television to present public issues in a way that allows for the nuanced presentation of diverse views and a good-faith interchange between advocates of opposing views. - Again, it misses the detail part. Even if we have advocates or opposing, each would be limited to 30 sec, promoting an oversimplified take. Moreover, while the argument says it "generally allows advocates of a point of view only 30 seconds to convey their message," - note GENERALLY doesn't necessarily rule out that TVs can't present a more nuanced approach. Distortion.
(D) In network television news reports, it is not usual for a reporter to offer additional factual evidence and background information to develop a story in which opposing views are presented briefly by their advocates. - Yes.
(E) Television news reporters introduce more of their own biases into news stories than do newspaper reporters. - "their own biases" is out of scope