The first alternative is correct (A).
By elimination, the other alternatives do not relate to the author's sentence. The author talks about a chapter with friendly and lively teens. This has nothing to do with the issue of employment for young people, nor does it suggest that they should be alone, nor does it refer to schools at all. However, the pleasant perspective of the adolescents narrated by the author better fits in the conception that adolescents are a pleasant part of the society.