The correct answer is (E).
(E) Assumption (Necessary)
Step 1: Identify the Question Type
The question asks directly for the argument's assumption. Start by identifying the author's conclusion and evidence, and determine the central assumption (the unstated premise, if you will) that will connect those two pieces of the argument. Since the assumption is "required," you can, if need be, use the Denial Test to confirm or eliminate answers.
Step 2: Untangle the Stimulus
The Keyword "[h]owever" indicates the author's opinion. He doubts that the syndicated columnists described in the opening sentence will achieve their goal of swaying readers' votes. His evidence (indicated by the Keyword "for") is that most voters have already formed opinions before the columnists' work appears.
Step 3: Make a Prediction
While voters may have formed opinions before reading the columnists' arguments, this evidence doesn't say that new information or arguments couldn't persuade them to change their minds. In order for the author to conclude that columnists won't persuade voters, he must assume that voters will maintain their points of view even after the release of the columnists' work.
Step 4: Evaluate the Answer Choices
(E) expresses exactly the author's assumption. The Denial Test confirms that this is a necessary assumption. After all, if people could be persuaded to change their minds, then the author has no solid reason why columnists couldn't succeed at persuading voters. His conclusion would fall apart.
(A) is irrelevant. According to the author, "nearly all who will vote" have already made up their minds. Influencing the undecided would have, at best, a negligible effect.
(B) suggests that columnists could accidentally have an effect opposite to their intentions. While this may strengthen the idea that they would be unsuccessful, it's hardly a necessary assumption. Even if they didn't have an adverse effect, they could still have no effect at all.
(C) might strengthen the argument by suggesting that voters probably wouldn't expose themselves to columns that could sway their opinions. However, it's not necessary for the author to assume this. It may well be that decided voters will read opposing views and still not change their minds.
(D) makes an irrelevant distinction between frequent and infrequent column readers, and thus doesn't adequately address the main issue: that almost all voters have already made a decision.
