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Okay, let's take this baby down. Many of the responses in this forum focus on isolated pieces of this question, but I think it might be beneficial to do a full analysis, focusing on the critical-thinking strategies that unlock problems like these. As you study for the GMAT, it is very beneficial to identify patterns and strategies useful for multiple questions, not just one. Here is the full "GMAT Jujitsu" for this question:

Our first item of business is to recognize the problem type. Fill-in-the-blank Critical Reasoning questions are notorious for obscuring what the question is asking. If you just look at the question stem, you might erroneously that this is an Inference question type, because of the phrase "most logically completes the argument." However, look at the structure of the argument. Right before the blank, the problem uses the word "since." "Since" is what I call in my classes a "Conclusion Conjunction." While it precedes a fact, that fact is directly attached to the conclusion. (For example, if the problem says, "X, since Y", then X is the conclusion. If the problem says, "Since X, Y", then Y is the conclusion. Thus, in this question, the answer choices are additional facts, supporting a conclusion up in the stimulus. The conclusion is "This fact [i.e., that fact that states with the strictest environmental regulations also have the highest economic growth] does not show that environmental regulations promote growth." The correct answer would undermine the idea that environmental regulations promote growth. You could think of this as a "Weaken" question.

There is a large logical gap here, related to the fallacy "Correlation is Not Causation." Even though the problem says that states with the strictest environmental regulations have the highest growth, we don't know if the environmental regulations caused the growth. (For example, what if a third factor -- such as an abundance of natural resources -- causes both growth and environmental regulations to happen? What if the causal relationship goes the opposite direction -- that high growth causes states to enact environmental regulations? There is no way to tell given the limited facts in the question!) The GMAT is hoping that, because it mentioned growth and environmental regulations in close proximity that you make the assumption they are related. If we can find an answer that shows growth might be caused by something else, we have our answer.

Answer choice “A” states that states with strict environmental regulations share another attribute: they invest the most in education and job training. Now, at first glance, this is a weak sauce answer. It never states that education and job training cause high growth. We have to insert in extra information in order to show that this new factor could truly prove the real causal link. However -- and this is crucial for those of you taking the GMAT -- "Strengthen" and "Weaken" questions on the GMAT aren't "Prove" or "Annihilate the Logic" questions. Notice the way the question is worded: it asks for the evidence that "most" logically completes the argument. Answer choice “A” does introduce other possible factors that could be connected to high growth. If all of the other answer choices fail to undermine the assumed causal link between regulations and growth, then this answer -- as weak as it is -- still "most" logically completes the argument. If this question showed up on my GMAT, I would reserve judgement on “A” until I looked at the other answer choices.

Answer choice “B” does the opposite of what we want, by reinforcing the assumed link between regulations and growth. If (1) moderate environmental regulations are correlated with higher growth than less-strict regulations are and (2) strict regulations are correlated with highest growth, the pattern seems clear. But we don't want to prove the pattern! We are trying to find an answer this shows the assumed link isn't necessarily there. “B” can be quickly eliminated.

Answer choice “C” is a beautifully-crafted trap. It says that "many states that are experiencing reduced economic growth are considering weakening their environmental regulations." (In other words, it sounds like the states are trying to fix their poor growth by reducing environmental regulations.) This seems to reinforce the idea of a linear relationship between growth and regulations. However, answer choice “C” suffers from the same problem that the original argument does: we assume that the states are "considering" weakening their environmental regulations BECAUSE they are trying to fix poor growth. But we don't know that. Remember: "Correlation is Not Causation." The states could be considering reducing environmental regulations for many reasons. Plus, we are looking for an answer that undermines the hypothesis that regulations "promote" growth. We know nothing about what would happen if we "reduced" regulations. You can't assume a linear relationship. (To give a parallel example, you could argue that eating ice cream all day could make you fat, but that doesn't necessarily mean that eliminating ice cream from your diet will make you skinny! Studies have shown that reducing calories can actually cause your body to go into starvation mode and try to build up emergency fat reserves.) “C” doesn't truly undermine the idea that strict regulations can promote economic growth.

Answer choice “D” also reinforces that assumed causal link. Get rid of it!

Answer choice “E” is completely irrelevant to the logical gap. Just because "states with very weak environmental regulations have experienced at least some growth," doesn't mean that stricter regulations couldn't promote even stronger growth. “E” doesn't undermine the assumed causal link. We can quickly eliminate it.

In the end, only one answer choice even gets us close to undermining the assumed causal link between environmental regulations and economic growth. Two answer choices ("B" and "D") do the exact opposite of what we want, strengthening the possibility of a causal link. Two answer choices ("C" and "E") don't mind the logical gap. These fail to come up with an alternative explanation for why states experience economic growth. Only one answer is left over. And it is total weak sauce. This is what I call in my classes a "Directional Nudge." With Strengthen and Weaken questions, test takers often look for an answer so perfect that Aristotle himself rises from the grave and gives them a round of applause. But that approach is a deliberate trap in many GMAT questions. Don't fall for it. Notice even in the question stem that it reads, "most logically completes". You just need the answer choice that does it best. And "A" is the only one that gets close.