In Europe, schoolchildren devote time during each school day to calisthenics.
Conclusion (We must conclude that)
North American children can be made physically fit only if they participate in school calisthenics on a daily basis.
Evidence
In Europe, schoolchildren devote time during each school day to calisthenics.
North American schools rarely offer a daily calisthenics program.
Tests prove that North American children are weaker, slower, and shorter-winded than European children.
Evaluate
This argument gives us two differences between European school children and American school children and draws a conclusion that seems to imply that one difference (school calisthenics) is the cause of the other (better performance on fitness tests). But the conclusion isn't framed as a causal explanation. Instead, it's framed as a conditional: the only way to achieve physical fitness is school calisthenics. Any time an argument concludes that something is necessary to achieve a result, we should be thinking about alternative ways to achieve the result.
Goal
Look for an answer that rules out an alternative way of achieving physical fitness without calisthenics, or otherwise establishes calisthenics as necessary. Typically, in Necessary Assumption questions, we want to be wary of strong answers. But because we're trying to support a conditional conclusion in this argument, a correct answer might use strong conditional language.