Since the new publisher took control, a news magazine’s covers have fe

Dear kanusha
First of all, let every one be advised:
for this question, the OA = (B).
I just changed the key at the top of the thread to reflect this.

Now, to find an assumption, let's use the negation test. First, let's look at (C) & (E), your choices.
(C) A magazine can boost sales while highlighting the coverage of important world leaders.
Negation = A magazine can't possibly boost sales while highlighting the coverage of important world leaders --- those two activities are mutually exclusive.
Well, the gossip columnist’s opponents' argument is: "the new publisher is more interested in boosting sales than in reporting important news events." If those two activities, boosting sales and covering important world events, are mutually exclusive, then this could strengthen that argument --- the publisher is much more interested in the former activity, and so neglects the latter activity.

(E) Magazine issues with models or movie stars on the covers are purchased at a rate more than three times greater than is the case with issues featuring politicians on the covers.
Negation = Magazine issues with models or movie stars on the covers are purchased at a rate less than three times greater than is the case with issues featuring politicians on the covers.
So, the rate is less than three times --- suppose it's 2.5 or 2.9 times --- that would not substantially change the argument. Any statement that makes some precise quantitative specification that is not demanded by the prompt cannot possibly be an assumption. As long as magazines with models or movie stars on the covers are purchased at some substantially higher rate than those with stodgy old world leaders on the cover, then the argument would hold. Even 30% more (i.e. 1.3 times) is a substantial difference in sales, and that's well below 3 times.

(B) Final authority for choosing the cover subject of the magazine lies with the publisher.
Negation = Final authority for choosing the cover subject of the magazine lies with someone other than the publisher ---- i.e., the publisher does not have final say over what's on the cover.
Well, if the publisher doesn't really get to say what's on the cover, then there's absolutely no way we could divine the publisher's intentions or priorities from what appears on the cover. What appeared on the cover would reflect the priorities of whoever gets to determine that, not the publisher. The negation of this idea absolutely obliterates the conclusion of the argument ---- we could not determine anything about what the publisher likes or doesn't like, prefers or doesn't prefer, from a cover that not the publisher but someone else decides. Since negating this choice creates a devastating objection to the argument, that's a sign that this choice is the true assumption.

Does all this make sense?
Mike