Premise: Genetically engineered crops do not need to be sprayed with insecticides.
Premise: Excessive spraying of insecticides has harmed wildlife.
Conclusion: Using more genetically engineered crops is likely to help wildlife recovery.
When we are asked for an assumption required, we need to find something that was not included in the text, but is absolutely necessary for the success of the argument.
The first thing I thought of after reading this argument was, "How do we know that the genetically engineered crops don't harm wildlife just as much as insecticides?"
If we don't know that genetically engineered crops are less harmful, then the argument falls apart. The author is assuming that they are less harmful, which is why answer choice A is correct.
On to answer choice B. This looks like a really good option until we get to the end. "Even slightly" is what makes B incorrect. At no point does the author suggest that a slight decrease in insecticides is enough to make a difference. The farmer might even believe that a dramatic decrease in insecticides is necessary for wildlife recovery. The argument stands nonetheless. We do not need to assume B in order for the argument to make sense.