This question is a little weak in that there seems to be a superfluous idea in what seems to be the right answer. Still, there is a best answer.

(A) While in a way the causes matter, the argument is not about why TV is watched more. It's about TV being the future. Would understanding the causes help to predict the future? Probably, but not having information on the causes does not totally undermine the argument.

(B) This is the best answer, and really it could be shortened to "The argument makes an assumption that the consumer preferences will not change significantly over time." The "trends" part of this answer choice does not really matter, as really the argument could consider trends or something else in making the case that consumer preferences will not change over time.

The key flaw in that argument is that it claims that TV is the future but supports that claim only by saying that TV is the king in the present.

(C) This is a typical trap wrong answer. Averages rarely apply to all the members of a set. An average is often the average of a variety of values, and for this argument to work the figures do not have to apply to all consumers of media.

(D) This answer could be construed as being similar to answer B, in that the population below 18 years of age will in the future be part of the population above 18 years of age, but while possibly the habits of those below 18 years of age could be somehow relevant to the validity of the argument, the simple fact that that population is not mentioned is not a key flaw in the argument. The key flaw is what B says.

(E) The argument does not even really mention any cause and effect pattern. So this is clearly out.

The correct answer is B.