Author's purpose asks why the passage exists: to explain, to argue, to describe, or to recount. It is wider than one paragraph's job. Main idea is the closest cousin, in main idea questions.
Whole passage, not one example
A choice that fits only the last anecdote is too small. Attitude is how the author feels, in author's attitude. What one sentence is doing is narrower, in function of a sentence.
Paragraph jobs are the middle scale, in paragraph jobs.
The bottom line
Purpose is the whole passage. Do not shrink it to one example.
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