Which of the following sentences requires commas to offset a nonrestrictive clause?

Question

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The woman who was speaking is Margaret Thatcher.
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Using anadiplosis, British Prime Minister Margaret Thatcher addressed the Conservative Party.
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British Prime Minister Margaret Thatcher used anadiplosis in an address.
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Margaret Thatcher who was the British Prime Minister used anadiplosis in an address.
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Answer 1

The sentence that requires commas to offset a nonrestrictive clause is:

Margaret Thatcher, who was the British Prime Minister, used anadiplosis in an address.

In this sentence, "who was the British Prime Minister" is a nonrestrictive clause providing additional information about Margaret Thatcher but is not essential to understanding who she is. Therefore, it should be set off by commas.