I have mixed feelings about this question.
- I find option B beautiful because it is ABSOLUTELY wrong but is very attractive for many people.
- I find option A a little difficult to digest. What works in favor of option A is that there is no other option that comes close.
Why is option B wrong?
Because it strengthens the argument rather than weakening it.
Before we evaluate how option B strengthens, it's important to be aware that it talks about "prepublication fact-checking," i.e., fact-checking that happens before the publication. The argument is about finding inaccuracies AFTER the publication.
Now, let's understand how option strengthens the argument: If magazine articles tend to go through less rigorous pre-publication fact-checking and still (given that) no critic has found any errors in these publications, Dennison is really accurate about his stuff. Even where pre-publication fact-checking is not very rigorous, nobody finds errors in his articles. In this case, if someone finds errors in Dennison's books, which have gone through more rigorous fact-checking before publication, we'll have a tendency to believe that the person finding errors is wrong. Thus, the conclusion will be strengthened.
Why is option A correct?
If books receive more reviews than magazine articles, I have a reason to understand why Dennison's magazine articles were not pointed out for inaccuracies while his books are. Since books receive more reviews, they are more scrutinized or probed. Thus, it's more likely that inaccuracies in the books will be pointed out.
Why did I have mixed feelings about A?
Because the argument says, "No critic has found any errors in these publications (magazine articles)." If the argument had said, "Very rarely, a critic has found any error in these publications," I would not have much difficulty in selecting option A.
Why?
Option A says that books receive more reviews than magazine articles, so this could explain why more inaccuracies will be found in books than in magazine articles. However, I could not bring myself to see how this option could explain the difference between "ZERO errors" in magazine article to "a number of inaccuracies" in books. Regarless of this concern, I agree that option A is the best.