The sentences that contain an example of contested usage are:
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"The postman needs to know who this letter is addressed to so it can be delivered." (The contested usage here is the preposition "to" at the end of the sentence. It is often debated whether it should be rephrased to say "to whom this letter is addressed.")
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"The chair for the chair of the board of directors is at the head of the board table." (The contested usage is the term "chair," which can refer to both a physical chair and the person in charge of a meeting or organization, leading to potential confusion over its intended meaning.)
So the answer is the third and second sentences in your list.