Thanks to everyone who voted. The majority, 68%, got it right. The incorrect punctuation mark was an apostrophe. Here is the question again:
Which punctuation mark is incorrect? The naughty boy—he broke a window—was sent to the principals’ office; he claimed he was innocent, though.
The apostrophe is incorrect because there is only one principal; therefore, it should be "the principal's office."
I want to ask those of you who voted otherwise why you thought your answer was the correct one (the other choices were em dash, comma or semicolon.
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