The Wheat Farmers Alliance, a political action committee, attracts 70

The Wheat Farmers Alliance, a political action committee, attracts 70 percent of its contributors from an advertisement requesting contributions placed only in the September, October, and November issues of the Grange Report, a monthly newsletter for wheat farmers. The president of the Wheat Farmers Alliance, to increase the number of contributors, decides to advertise in each of the monthly issues of the Grange Report. She expects that, as a result of the additional Grange Report advertisements, the number of contributors will be increased to at least double the present number.

Which of the following, if true, would most strongly support the president's expectation?

(A) The September, October, and November advertisements were noticed by fewer than one-third of those readers of the Grange Report who would be willing to contribute to the Wheat Farmers Alliance. - CORRECT. The reach of advertisements increase with increase in the advertising months, eventually leading to higher probability of getting more contributors.
(B) Wheat farmers traditionally repay their bank loans in late summer after the winter wheat crop has been harvested and sold. - WRONG. Irrelevant at best OR more assumptions needed to be made for it be a good choice.
(C) The majority of the readers of the Grange Report with a great enough interest in the Wheat Farmers Alliance to contribute have already responded to the advertisements. - WRONG. Weakens instead.
(D) Most of those who contribute to the Wheat Farmers Alliance in the course of a year do so in response to advertisements in the Grange Report. - WRONG. Similar language but different result. It results in increase in contributions for the set when instead we seek the increase for its subset.
(E) The total number of readers of the Grange Report is stable from year to year. - WRONG. Yes, then what!! How it would lead to more contributions? Not clear enough from this choice.

Answer A.

Please help me understand why 'A' & not 'D'?

Let's first consider (D).

(D) Most of those who contribute to the Wheat Farmers Alliance in the course of a year do so in response to advertisements in the Grange Report.

The easiest way to eliminate (D) is to notice that the passage says the following:

The Wheat Farmers Alliance, a political action committee, attracts 70 percent of its contributors from an advertisement requesting contributions

Since "70 percent" is "most of those who contribute," (D) simply repeats what the passage has already stated. We can't strengthen an argument by with information that has already been presented.

Now, let's consider (A).

(A) The September, October, and November advertisements were noticed by fewer than one-third of those readers of the Grange Report who would be willing to contribute to the Wheat Farmers Alliance.

This information confirms that there are enough additional readers who would contribute for the present number of contributors to be doubled. With the question of whether there enough additional readers answered in the affirmative, we have more confidence that the plan will work. In other words, by eliminating the possibility that there are not enough, we've increased the probability that the plan will work.

Still NOT convinced with the reasoning to eliminate D.

I was able to narrow it down to A and D.

But Not Sure why to eliminate D.

GMATNinja, KarishmaB,
please help.

The main point of the argument is that instead of advertising in only Sep, Oct and Nov issues, the plan now is to advertise in all 12 issues of the year. The aim is to at least double the number of present contributors.
So the point of distinction will be advertising in only Sep/Oct/Nov vs advertising in all months.
We need to support that the plan will succeed; that more people will contribute if advertisements are placed throughout the year.

(A) The September, October, and November advertisements were noticed by fewer than one-third of those readers of the Grange Report who would be willing to contribute to the Wheat Farmers Alliance.

So one third of the report readers willing to contribute noticed the Sep/Oct/Nov ads. So if ads are placed all year, likely more willing to contribute readers will notice them. Hence, the contribution will likely increase if ads are placed in all months.
This does help our plan.

(D) Most of those who contribute to the Wheat Farmers Alliance in the course of a year do so in response to advertisements in the Grange Report.

We have already been told this in the argument: "The Wheat Farmers Alliance attracts 70 percent of its contributors from an advertisement requesting contributions ... issues of the Grange Report"

So we know that 70% contributors see the ads and then contribute. So most of those who contribute do so in response to the ads.
This does not make our plan more likely. It gives us no point of distinction between why placing the ad in all issues will be more beneficial that placing it in only Sep/Oct/Nov issues.

Answer (A)

Thanks KarishmaB for elaborating.
Now its much clear and I got the explanation from MartyTargetTestPrep (poor me, could NOT get it earlier).