In a certain university course, some of the students

[asker]

In a certain university course, some of the students took their own handwritten notes on the lectures, while others studied from a printed set of notes written by the course instructor; no student used both kinds of notes. The students who took their own notes performed substantially better on the course exams than did those who used the notes written by the instructor. Clearly, the students' own notes must have been more complete or easier to understand than the instructor's, or else the students who took their own notes were simply those who were more motivated to do well in the course.

Which of the following statements, if true, most weakens the argument in the passage above?

a In other classes at the same university, students who both wrote their own notes and used the pre-printed instructor notes were among the highest-performing students on class exams.
b The exams were not written by the course instructor.
c Studies have shown that the physical act of note-taking aids in the recall of what is noted, even for those who never review the notes.
d The instructor's notes were written in language that was so florid and wordy as to make the points difficult for students to comprehend.
e Students in the course were not allowed to refer to any notes, whether their own or the instructor's, while taking the course exams.

OA: C

I was wrongly chose C:

I was thinking: "When compared with D, realized it's a tricky option: "studies showed...", what studies? can it represent the real case here? But D clearly talks about the case of inst in this specific university, so D"

Then when I read OE, it said: '(D) If this statement is true, then it supports the first explanation provided in the passage, by substantiating the hypothesis that the students' own notes were easier to understand. This would strengthen the conclusion."
Then I realized how I was wrong. Then my question will be, what if the statement above changed to the following statement:

"
In a certain university course, some of the students took their own handwritten notes on the lectures, while others studied from a printed set of notes written by the course instructor; no student used both kinds of notes. The students who took their own notes performed substantially better on the course exams than did those who used the notes written by the instructor. Clearly, the students who took their own notes were simply those who were more motivated to do well in the course.
"

Keep the weaken question and all the options the same, the will D be better than C, as I analyzed?

[Chelsey Cooley]

In that case both D and C would be entirely correct, so it wouldn't be a fair question. A 'weaken' answer doesn't have to be totally airtight (like an 'assumption' or 'conclusion' answer might); it just has to give a very clear suggestion that the conclusion might not be right. In your edited example, both D and C would do that by proving that there could reasonably be another reason the students did well.