The fact that people who exercise vigorously are sick less often than average does not prove that vigorous exercise prevents illness, for whether one exercises vigorously or not depends in part on one’s preexisting state of health.
Conclusion
The correlation between vigorous exercise and infrequent sickness doesn't prove that vigorous exercise causes the absence of illness.
Evidence
(It could be reverse causality). Maybe "not being sick" is a precondition for doing vigorous exercise.
Evaluate
When we do Parallel questions, we're initially thinking - is there conditional logic / quantifier stuff here? if so, let's turn it into Algebra. - if not, or if we already have some sort of reasoning pattern vocabulary to describe it (like a Famous Flaw or a Method of Reasoning answer), we'll use that. - if not, we'll try to think about what sort of Principle the author is using.
In this case, we're supposed to be thinking,
If this were going to be a flawed argument, the author would present a correlation between exercise and health and then conclude that the exercise causes the health (while failing to consider alternative explanations, such as Reverse Causality — maybe the health causes the exercise — or Third Factor — maybe wealth is what allows us to stay in good health and to have the disposable time to exercise).
Since this isn't a flawed argument, the author is doing the responsible thing LSAT wants us to do with correlations -- saying that a correlation doesn't prove one thing causes another, because it could suggest a cause/effect relationship in reverse as well.
Goal
Look for something resembling this: - The fact that X and Y are correlated does not prove that X causes Y - maybe Y comes first (maybe X depends on Y)