A recent study found that individuals who engage in 30 minutes of moderate physical activity five times a week report a 20% improvement in mental well-being compared to those who do not exercise. Critics argue the study may overstate exercise's benefits as it relies on subjective self-reported data and ignores other factors like diet, sleep, and social interactions. Despite these concerns, supporters claim the findings align with previous research linking physical activity to better mental health.

Which of the following, if true, would most weaken the conclusion that physical activity significantly improves mental well-being?

Option C : The follow-up study demonstrates that other factors, such as diet and sleep, can produce similar mental well-being improvements, suggesting exercise is not the primary determinant of better mental health. This directly challenges the study's conclusion.

Option A: Strengthens the argument by providing additional support from previous research.

Option B: Points to a sampling bias, which is a limitation but does not directly refute the study’s conclusion.

Option D: Highlights a methodological issue but doesn’t negate the link between exercise and mental well-being.

Option E: Suggests another factor, social activity, might influence mental well-being, but it does not directly undermine the conclusion that exercise plays a significant role.

Hence, the answer is Option C.