Going with A.
Conclusion: If you play sports, you will be fit and will prevent sickness.
We need to prove the conclusion false. This is a common ETS trick. Use the two pieces in the conclusion to make the conclusion false.
That is, what if you are already sick or disabled that would prevent you from playing. Then the benefits from playing sports would not matter.
A school nurse noted that students who participated in sports were sick less often than those who did not participate in sports. The nurse concluded from this that participation in sports not only promotes fitness but it also prevents illness.
Which of the following, if true, most seriously weakens the argument above?
(A) Illness, especially regular illness, causes physical effects that prevent participation in most physical fitness regimens and most sports.
(B) Students who exercise regularly get sick less often and suffer from less virulent illnesses than do students who do not exercise regularly.
(C) Illness does not result in a state of lethargy, which prevents those so afflicted from leaving the home except for the most important activities.
(D) Students who are ill are not allowed to go to school for fear that they will pass the infection to schoolmates.
(E) Students who are not physically fit are more susceptible to injury and infection.