Okay! This is my first time answering someone's question online. If there is anything wrong then I hope someone will point it out for me. Thanks in advance!

Yes, this is a necessary assumption question.

I think the key to the answer is the author's 'only if' conditional in the conclusion. If daily calisthenics is the only way to achieve physical fitness then:

physically fit ---> daily calisthenics

Basically the author's assumption is that there is no other way for a child to achieve the goal of becoming physically fit other than by doing daily calisthenics (answer choice D). If you fail that necessary assumption (that daily calisthenics is not the only way for children to achieve physical fitness) then the argument fails.

Whereas in (A) it is saying:

child --> could be fit by daily calisthenics

Answer choice (A) is saying that if a child does daily calisthenics then they can get fit. According to this answer choice, it's a possibility that they become physically fit, it's not a necessity. So who does get fit and who doesn't? According to the author conditional statement in the conclusion, those who [successfully] do daily calisthenics (affirming the necessary condition).

In the stimulus, the author does talk about how successful children are at calisthenics (which is what answer choice A is talking about). For the argument to be valid, the author does not have to assume that every child who does daily calisthenics is going to be successful, just that those who are successful will have achieved physical fitness.

How do we know? Well, let's fail the assumption that every child who does daily calisthenics has to be successful. Does that result in making the argument fail? No, all it does is tells us that those who failed wouldn't have achieved physical fitness, but those who were successful would have achieved physical fitness.

The author's assumption is not that anyone who tries to do daily calisthenics will be able to do it and as a result achieve physical fitness. Rather, the assumption is that if the children achieved physical fitness then then they did so by successfully doing daily calisthenics (affirming the necessary) which, in the author's opinion, is the only way to attain physical fitness:

physically fit ---> daily calisthenics

I hope this makes sense :neutral:

In Kaplan's Guide to this LSAT there is a note for this question before the explanation that says: "The Law Services official answer key may read (B) for this question, but this is a typo. The correct answer is (C)."