The chemist’s conclusion is about the reliability of the data; we need an answer choice that suggests that the data is indeed misleading. (A) simply reinforces the fact that one of the weed-killer’s molecules kills weeds and the other doesn’t.

This suggests that the data is indeed misleading. If the data comes from lab studies where both molecules are equally concentrated and likely to break down, it doesn't reflect how the weed-killer works in local soil, where one form usually breaks down more than the other.

Even if the weed-killing molecule is more concentrated in most local soil conditions, this tells us nothing about whether the data is misleading. Did the data account for the fact that this molecule is more concentrated in most local soil conditions?

This weakens the chemist’s argument. If the data was drawn from studies that used a variety of representative local soil conditions, then it’s less likely that the data is misleading.

It’s true that data that examines just one form of the molecule is likely misleading. However, we don’t know if the data in the argument makes this mistake. It might examine both forms, in which case (E) doesn’t tell us anything about its reliability.