IMO D.
A recent study of college students shows that, contrary to predicted results, special nutritional planning does not positively affect students' grades. Sixty students, half of whom were given a nutritionally balanced diet, had grades no higher than did those students who were not placed on the diet plan.

Which of the following, if true, is most useful in determining the accuracy of the study described above?

What we have to notice here is that there was a study conducted earlier based on which there is a prediction that the grades of the students should increased on having a nutritionally balanced diet.

As D supports the prediction of the study, I think its D.

Did you transcribe this correctly? This is not a good question.

The language here is not good. "Sixty students, half of whom were given a nutritionally balanced diet, had grades no higher than did those students who were not placed on the diet plan."

It should say, "The half of the 60 students who were given the nutritionally balanced diet had grades no higher than those students who were not placed on the diet."

With that said, I am not a fan of the answer choices here.

I see that "winniethepooh" goes for D and "gmat 25" goes for B. I think that each of these choices has problems.

For B I am not sure what "honors students" had to do with it. If this choice said that those students who changed their diet were also forced to reduce their hours studying that would be significant because it would show that the nutrition changed and the studying did not so that we are measuring only one variable. Could this be a typo??? If B said "College students in this study, after altering their diets..." then this would be a decent choice because that would place this study into context.

I checked and B is not a typo. It is just a bad answer choice.

I just looked and the official answer is supposed to be D based on another post. This is not a good question or answer. We are asked to determine the accuracy of the study above. The study above is college students. The answer choice is college students. Different ages, different brain chemistry, etc. Not comparable. Also, why would that one study (high school) necessarily show that this study (college) is not accurate.

How to approach this one?...skip it.

@David

Thank you so much for replying. I completely agree with you that both the choices had problems. Before posting the question, even i had the same point of view as yours but still thought that an experts comment will gonna be really helpful so i pm'ed you.

The official explanation is pathetic but still i post the same here.

Explanation says, its falls in the category of "evaluate the argument"

for Op B, its wrong because: The study considers how diet affects students' grades, not their study habits

for Op D, its correct because: This is also a study of how diet affects students' grades and could be compared to the study mentioned in the argument.


Now i have one question here, is it really an "evaluate" type question. I thought its moreover a kind of strengthening question where we need to provide extra premise in order to validate the study...m i right?? that's why i do believe that if we remove "honors" in Op B, it can be a correct answer choice. What's your opinion here. As u suggested, i have no interest in discussing this question, i'm only interested to know did i interpret the question stem correctly???????

Sorry to bring this one up from the graves. But need expert reply on this one plz.

Option C does the trick here.
According to it, the grades improved by an X process. It is possible the grades may just not improve. Could be that sudents were not willing to improve.

Option C proves that the grades could be improved, but by nutri diet they did not.
Hence we need C.