We can see that the last part of the sentence above can be improved by adding: D. was provocative, well researched, and insightful.
Improvement actually refers to the process of make something better. It refers to enhancing something and making it acceptable.
We can see that if the given sentence was actually improved on, it will become:
We appreciated the article you wrote for the school newspaper because it was provocative, well researched, and insightful.
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