Understanding the argument -
Goal - increase their income significantly.
Plan - Cease growing soybeans and begin raising cotton instead.
What is this plan dependent on? Cotton prices are rising. How? Because of Insect infestation. Insect infestation (cause) leads to high cotton prices (effect). And we made our whole plan on this. Oh, I see. So this argument is like a bunch of cards stacked on top. You pull one key card, and the whole stack comes down. What is that key card here? The insect infestation. If somehow the insect infestation comes down, then the argument falls apart.
There could be other weakeners as well. What if the soil doesn't support cotton? What if, as the supply increases, the prices immediately come down? But this argument uses such a fundamental weakness that it's easy to miss this weakness.
Option Elimination -
(A) The cost of raising soybeans has increased significantly over the past several years and is expected to continue to climb. - Strengthener.
(B) Tests of a newly developed, inexpensive pesticide have shown it to be both environmentally safe and effective against the insects that have infested cotton crops. Ok. It beautifully takes out the key card, and the whole argument crashes.
(C) In the past several years, there has been no sharp increase in the demand for cotton and for goods made out of cotton. - We are now discussing the issue of insects and what happens in the future, out of scope.
(D) Few consumers would be willing to pay significantly higher prices for cotton goods than they are now paying. - This is a trap. Use your sentence correction hat. Basically, this argument says no consumer is willing to pay higher than they are paying now. E.g., If they are paying an increased price of $100 per unit (say it was $10 per unit earlier than infestation). So this question says no one or few will pay more than 100, say 110. Soybean growers don't care, as they are already getting $100
Which is pretty handsome than $10. Distortion.
(E) The species of insect that has infested cotton plants has never been known to attack soybean plants. - out of scope.