Not GMAT, but let's try it out for fun!

The question here reminds me a lot of GMAT Method of Reasoning question. Yes, the "most vulnerable" part makes it like a Weaken, but really we're being asked to identify HOW the reasoning proceeds. Method of Reasoning questions focus on how an argument is attempted, contradicted, or put forth. Unlike Strengthen or Weaken questions, you don't need to focus on the merits of the argument so much as its argumentative strategy!
Some people believe that good health is due to luck. However, studies from many countries indicate a strong correlation between good health and high educational levels. Thus research supports the view that good health is largely the result of making informed lifestyle choices.
The reasoning in the argument is most vulnerable to criticism on the grounds that the argument
ARGUMENT: Luck --> Health. Health <--> Education. So, Lifestyle --> Health.
This argument is pretty bad. It makes a strong cause/effect claim, based on a simple correlation. The assumption is that correlation between health/education means that lifestyle causes health. Futhermore, it makes a big jump between "education" and "lifestyle choices."
WHY this argument is bad is going to be our prediction.
PREDICTION: It assumes "education" = "lifestyle choices" AND that correlation = causation. By correlation = causation, we mean that an author believes that just because two things are connected (correlated), they have a causal relationship. To weaken, we can show that the correlation is mere coincidence, that the causal relationship actually happens in reverse (X causes Y instead of Y causing X), or that there is an outside cause for both things.
A. presumes, without providing justification that only highly educated people make informed lifestyle choices
B. overlooks the possibility that people who make informed lifestyle choices may nonetheless suffer from inherited diseases
C. presumes, without providing justification, that informed lifestyle choices are available to everyone
D. overlooks the possibility that the same thing may causally contribute both to education and to good health
E. does not acknowledge that some people who fail to make informed lifestyle choices are in good health
FIRST PASS:
A - maybe
B - "inherited diseases" is far too specific/out of scope
C - "available to everyone" is not really within scope
D - maybe; there could be a 3rd thing
E - why does it need to acknowledge this?
SECOND PASS:
It has to be between A and D. Between these two, D must be correct since it attacks the LARGER reason why the argument is flawed. It falls into the classic "correlation/causation" trap. Just because two things are found to occur together, DOES NOT MEAN that one causes the other. A third thing that is unforeseen could be causing both of them.
The answer is D.
Veritas has a nice little blog with another Causation/Correlation question:
https://www.veritasprep.com/blog/2015/03 ... questions/
Khan Academy also dives into this concept:
https://www.youtube.com/watch?v=ROpbdO-gRUo
Hope this helps!
Best,
Vivian