The press reports on political campaigns these days as if they were ch

Yes, the last part ("let the press report on the most revealing positions on substantive issues the candidates have taken") clearly states the same.

The source of this question does not seem to be authentic.

One may think (B) is simply the restatement of the argument but its not:

Even though answer choice (B) appears as though it were already stated, look closely at the language. It reads:
"... let the press report on the most revealing positions on substantive issues the candidates have taken."
This statement only refers to the most revealing statements they have made, but doesn't actually claim they've made any. If I were, for example, to claim that the press should report on the most accurate statements made by a particular candidate, yet the candidate only tells lies, then I can't argue the press should really be focusing on any statements.

The same is true here. If the candidates haven't actually taken any positions on the issues, you can't really focus on the most revealing of those positions. So, for this argument to make sense, the author had to assume that there were at least some positions that were taken.

The press reports on political campaigns these days as if they were chess games. One candidate’s campaign advisor makes a move; the other candidate’s advisor makes a countermove. The press then reports on the campaign advisors and not on the candidates. The losers in this chess game are the voters. They are deprived of the information they need to make informed decisions because the press is ignoring substantive policy issues and reporting only on the process of the campaign. It is clear that the campaign advisors should stay out of the limelight and let the press report on the most revealing positions on substantive issues the candidates have taken.

Which one of the following is an assumption upon which the argument in the passage depends?

(A) Chess is the most appropriate analogy to reporting on political campaigns. - WRONG. Irrelevant to know about this.

(B) The candidates in the election are taking positions on substantive policy issues. - CORRECT. POE helps. At first it does not look worthy but other choices are relatively bad. If candidates are not taking positions than passage stands to lose.

(C) How the press reports politics determines the substantive issues in the campaign. - WRONG. It does not but something else is substantive unless the process is suggested in reports to be substantive.

(D) The voters are not paying enough attention to the election to be able to make informed decisions. - WRONG. Irrelevant.

(E) There is no difference between reporting on the political process and reporting on substantive issues. - WRONG. Opposite to what is stated in the passage.

Answer B.