The correct answer is D

Generalize:
g-mod cotton - resists insects
farmers used-not much diff in quantity of insecticides-
switching makes little sense

Here we want to understand why the farmers said that the gap in insecticide usage was small. There are two possibilities.
1. They overused insecticide since this was the first yield
2. The g-mod insecticide and regular cotton insecticide were sprayed on two different size fields.
3. Cost difference - I doubt such an easy answer exists in the GMAT!

Lets see why D is correct by proving others incorrect:

A. Whether farmers who tried the modified cotton seed had ever tried growing other
crops from genetically modified seed - Ok. Lets assume that they have grown other crops. THis does not affect cotton's insecticide usage.

B. Whether the insecticides typically used on ordinary cotton tend to be more
expensive than insecticides typically used on other crops - Ok. Lets say that price of insecticide for ordinary cotton is more. This would have reduced the gap in cost savings even more.

C. Whether for most farmers who grow cotton it is their primary crop - Lets assume yes. Does this help in evaluating the cost? No

D. Whether the farmers who have tried the modified seed planted as many acres of
cotton, on average, as farmers using the ordinary seed did - Yes! If I took only one acre of regular cotton and my friend had 10 acres of g-mod cotton, obviously the cost of insecticide for me will be very less compared to his cost. If we both had 10 acres then we could easily evaluate the cost of insecticide and resolve the doubt

E. Whether most of the farmers who tried the modified seed did so because they had
previously had to use exceptionally large quantities of insecticide-Careful! Read the sentence correctly! 'Whether the farmers who tried the modified seed did so'...........
Did so? Did they try to plant seeds because they had used large quantities of insecticide? This sentence is very vague and compares planting seeds to using insecticide. Between D and E, E sounds smart but vague.