Answer. The final sentence:
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"The salesperson handed a receipt to each customer that was very long. "
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→ is the sentence that contains a misplaced modifier.
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→ Specifically, the placement of the phrase: "that was very long".
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This is a misplaced modifier, because it is difficult to determine if "very long" modifies either: 1) the actual "receipt" ; or, 2) the "customer(s)" —specifically, "each customer [that happened to be 'very long"].
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