M: It is almost impossible to find a person between the ages of 85 and 90 who primarily uses the left hand.
Evaluate
Neither person provides an argument. Each person just provides one idea.
The question stem, meanwhile, is probably the most confusing task I've ever seen on a logical reasoning question stem.
The answer is supposed to be a hypothesis (causal explanation) someone might offer on the basis of M's fact.
The word "hypothesis" almost always means causal explanation, in the Explain Curious Fact sense of the word.
Q's claim is supposet to counter / weaken this hypothesis. We know that when we are weakening an Explain Curious Fact style argument, we usually do so by offering an alternate explanation for the curious fact, and sometimes do so by undermining the possibility of the hypothesis that the author is selling.
So let's pretend that we are an LSAT author, and come up with some causal explanation conclusion for M's premise.
The curious fact that M presents is,
This is essentially a correlation between being very old and being right handed. An LSAT author would typically assume that one of those things is causing the other. So someone might jump to the hypothesis that "being right handed makes you live longer", that would explain why the really old people are right handed.
Would Q's response sound like an alternate explanation for why super-old people are right handed, or would it undermine the possibility that right handedness causes you to live longer?
Yes! Q's response would sound like an alternate explanation. She would be saying,
In other words, a lot of of these 85 to 90-year-olds who primarily use their left hand are actually left-handed! They were just trained/shamed as children into learning how to do everything with their right hand.
Goal
We want an answer that makes it sound like left-handedness causes you to die earlier or right handedness causes you to live longer, so that Q's response can feel like an alternate explanation for the curious fact.