The phrase "most calls into question" lets us know that this is a WEAKEN question. What is the core of the argument?
The CONCLUSION in this case, is the conclusion of the soybean farmers implementing the plan:
to increase income significantly over the next few years, they will grow cotton instead of soybeans
The PREMISES given to support this conclusion are that (1) the price of cotton has risen dramatically lately, due to insect infestations, while (2) the price of soybeans has remained stable.
If you've been working on a lot of CR problems, or just have a knack for them, you might have instinctually identified some weaknesses in the reasoning right away. If so-- great-- having something in mind when looking at the answers--even if that something isn't the exact answer you end up with-- helps you stay focused.
Just because cotton prices have risen, does that mean that the farmers growing cotton have made more money? Perhaps they had to spend much more to fight the insect infestations. What are the typical profits on soybeans and cotton to begin with? Sales price is only part of the equation. Also, the price has increased dramatically, but will it sustain at these high levels? There are many assumptions being made.
If you didn't see flaws right away, that's okay too--it is even more important to be really specific about the exact wording of the conclusion and premises when looking at the answers. Examine each choice ruthlessly for even the smallest word that could rule it out-- you're looking for what's SPECIFICALLY WRONG, not what's right.
(A) If the prices of soybeans are rising significantly then this might seem to suggest that the farmer's are wrong to switch from soybean to cotton because cotton prices have increased. But there are a lot of variables that remain unknown (how big is the increase? how sustainable is it? what are the profit margins on both?)...it's unlikely to be our answer, but if you want to hold onto it for now that's cool--just rememeber there are some issues.
(B) A new super-pesticide (Cheap! Environmentally friendly! Safe!) might well knock out the insect infestation that has caused the price of cotton to increase. Prices could fall back to pre-infestation levels, eliminating the primary reason farmers were planning to switch--higher prices due to insects. This is a great candidate for our answer, and beat outs A. It would be safe to eliminate A at this point and hold B as our main contender. But keep going!
(C) If prices have increased while demand remained the same, then increasing the supply of cotton (by these new farmers switching) might decrease the demand, and therefore the price. BUT this choice tells us what has happened in the PAST rather than giving us information that will have an effect on the future. And notice that choice B DIRECTLY attacks the premise that is the basis for the farmer's plan, while choice C requires some bigger hypothetical leaps. Eliminate.
(D) If anything, this choice would strengthen the farmer's plan (albeit to a very small degree). Eliminate.
(E) You might make the leap that if this choice were true, the farmers would be "safer" with soybeans, but look closely. If the farmers are choosing to switch because of the infestation, then saying that the infestation doesn't affect soybeans doesn't give us any new useful information (they were already planning to switch because the infestation changed the cotton market, while their market was unaffected). Eliminate.
Although many of the distractors will seem to weaken, you must pay attention to the very specific wording and underlying structure of the argument, and choose the answer that most directly affects those things. The answer is B.