From Ron神：try to diagram the argument, numbering each of the sentences and using the 'THEREFORE TEST' (try 'x, therefore y' and 'y, therefore x', and see which, if either, of them makes sense).

1 Plant scientists have used genetic engineering on seeds to produce crop plants that are highly resistant to insect damage.
2 Unfortunately, the seeds themselves are quite expensive, and the plants require more fertilizer and water to grow well than normal ones.
3 Thus, for most farmers the savings on pesticides would not compensate for the higher seed costs and the cost of additional fertilizer.
4 However, since consumer demand for grains, fruits, and vegetables grown without the use of pesticides continues to rise, the use of genetically engineered seeds of this kind is likely to become widespread.

the structure of this argument is:
1 = background fact
2, therefore 3
however, 4 (which weakens conclusion 3)

if you figure out that much, then it's clear that #3 is the CONCLUSION. (that's the single most important thing you can do on critical reasoning: figure out what's the conclusion.)

since neither of the boldface portions is the conclusion, choice a and choice d are dead. (both of those claim that the second part is the conclusion.)

now, analyze the boldface parts a little better:
#2 (the first boldface) serves as a premise for #3
#4 (the second bold) serves as a counterpoint to #3, weakening the lasting value of the conclusion
...which is basically what choice c says.

choice b is tempting, but you should note that the argument is specifically designed to point out that this 'state of affairs' CAN affect the outcome. choice e is nonsensical, as the two boldfaces are on completely opposite sides of the proverbial fence.