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Hello everybody. This question drives me crazy
Premise:
1.Several studies showed that when participating in competitive sports, people who have recently been experiencing major stress in their lives are several times more likely to suffer injuries than other participants in competitive sports.
2. Risking seriously is unwise
Conclusion: No sports activity should be used as a method for coping with stress
Question: Which one of the following principles, if valid, most helps to justify the reasoning in the advice columnist's argument?
And right answer choice:
If people recently under stress should avoid a subset of activities of a certain type, they should avoid all activities of that type.
The only way I can make any sense from this answer is by equating "risking serious is unwise" to "should avoid a subset of activities". But this is such a huge leap! Jumping from evaluation (unwise) to prescription (should avoid).

What do you think?

.

Thank you Daily_Double, your explanation definitely makes sense.
I tried to track my real time thought process and arrived at this:

1.While reading stimulus I spotted “Several studies showed” that almost always leads to trouble of overgeneralization. And this argument indeed made overgeneralization from few studies to prescription to avoid all sport activities.
2.The second flaw that I spotted was "risking serious is unwise" to "should avoid a subset of activities" gap.
3.And finally I spotted leap from "avoiding competitive sports," to "no sports activity". What is strange that if all sports are competitive by definition this leap actually is not a flaw. Just from curiosity I typed in Google “definition sport” and first result it gave me was “an activity involving physical exertion and skill in which an individual or team competes against another or others for entertainment”.

LSAT directions say that “You should not make assumptions that are by commonsense standards implausible, superfluous, or incompatible with the passage”. But I can’t tell for sure that by commonsense standard there are non-competitive sports. For sure, in the context of the argument alone, competitive sports are represented like subset of sport, but from the wider perspective it is not necessary true. So I decided that maybe this is kind of “double agent” flaw (friend on surface and enemy inside) and it ended up that I was wrong .

Resume:
Lesson of The Day – consider argument ONLY in the context of argument alone without bringing in real world controversies.