Good Question !

In the past, farmers used ordinary seeds in their farms, along with the insecticide to protect it from insects.
Now, last year few farmers used genetically modified insect resistant seeds, which are costlier than ordinary seeds. As seeds are already insect resistant, it require lesser or no amount of insecticide, lowering the cost on insecticides. Still, few farmers used insecticides.
Conclusion=> The above change is unlikely to increase profit.

Analysis:
In general, Profit= Sale- Cost.
As argument doesn't mention about sale, assuming its constant. So, profit in inversely proportional to cost. If cost is incresed, profit would decrease.
Old cost =cost(ordinary Seeds) + cost(insecticide)
New cost=cost(Genetically modified seeds) + cost(insecticide)

As in second case, genetically modified seeds are costlier, extra spending done on costlier seeds needs to be compensated by reducing the cost on insecticide.

For example,
Old cost = 10K(seeds) + 2 K( insecticide)
New cost= 11K (genetic seeds) + Cost of insecticide.

In second case, Farmers' spending of insecticide should be less than 1K for profitable business.

(A) Whether there are insect pests that sometimes reduce feed-corn yields, but against which commonly used insecticides and the genetic modification are equally ineffective. Contradicts the premise itself saying insecticides and modified seeds are ineffective.
(B) Whether the price that farmers receive for feed corn has remained steady over the past few years. price of corn is out of context.
(C) Whether the insecticides typically used on feed corn tend to be more expensive than insecticides typically used on other crops. Concern is about amount of insecticide, and not cost of insecticide
(D) Whether most of the farmers who tried the genetically modified ed corn last season applied more insecticide than was actually necessary. Correct, As genetically modified seeds are already insect resistant, more usage of insecticide will reduce the profit.
(E) Whether, for most farmers who plant feed corn, it is their most profitable crop. Its obvious truth that corns are profitable. This doesn't touch the premises itself.