An author's-purpose question asks what the writer is doing, not what happened in the story. The answer is a job: to illustrate, to contrast, to qualify, to introduce. Function questions are the same family, in function questions.
Name the job
If the sentence is an example, the purpose is to illustrate. If it turns with however, the purpose is to contrast. Pivot words mark those jobs, in pivot words. First and last sentences of the paragraph are the usual home of the purpose, in first and last sentences.
A choice that retells the plot is a detail, not a purpose. Detail questions are in detail questions.
The bottom line
Answer with a verb about the writer: show, contrast, warn, define. If the choice only restates the content, it is the wrong question type.
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