Happy to walk through the differences between (A), (C), and (D). Let's start first by going over the stimulus.
Sheila states that health experts generally agree that smoking is bad for your health. Tim disagrees and cites his grandfather as evidence. His grandfather smoked cigars from 14 all the way to his death at 96.
The question says, "A major weakness of Tim’s counterargument is that his counterargument..."
Had Sheila said something about everyone in history who has smoked is bound to suffer from poor health, then Tim's example of his grandfather would provide for a strong counterpoint. However, this is not what Sheila says, as she simply states that smoking tobacco for many years is really likely to cause poor health.
If you play the lottery, you're very unlikely to win the jackpot. However, some people do win the jackpot. The fact that some people win the jackpot does not mean that you're no longer unlikely to win the jackpot.
In the stimulus, just because Tim has provided one example of someone who smoked for many years without having poor health doesn't mean that Sheila's statement that smoking tobacco for many years is very likely to be harmful to health. Her statement is still just as strong because she said "likely," and "likely" leaves room for exceptions.
(C) says, "describes an individual case that is explicitly discounted as an exception to the experts’ conclusion"
The issue with (C) is that there isn't anything in either Sheila or Tim's statements that explicitly discounts Tim's grandfather as an exception to the experts' conclusion. In order for (C) to have been correct, one of them would have to have mentioned something about elderly people who've smoked for decades and lived a long life not counting, but this is not stated anywhere. Thus, (C) is descriptively inaccurate, so we can get rid of this choice.
(D) says, "presupposes that longevity and health status are unrelated to each other in the general population"
The problem with (D) is that Tim doesn't presuppose that longevity and health status are unrelated to each other in the general population. Tim's argument is that smoking has no effect on health. He cites his grandfather as an example, saying his grandfather smoked for decades and lived until 96. He's using his grandfather's lifespan as a proxy for good health. In doing so, Tim is actually implicitly assuming that health and longevity are RELATED, not unrelated, to each other.
(A) says, "attempts to refute a probabilistic conclusion by claiming the existence of a single counterexample"
This is exactly what we're looking for. The probabilistic conclusion Tim is refuting is Sheila's claim that smoking tobacco for decades is likely to cause harm to one's health. Tim brings up the existence of a single counterexample, his grandfather, to refute Sheila's conclusion. (A) matches our anticipation of the weakness in Tim's argument really well, and it's the correct answer choice.
Does this make sense? Let us know if you have any more questions!
