The correct option is C A single letter isn’t misplaced by the postman.
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The correct answer is ‘A single letter isn’t misplaced by the postman.’
In the given sentence, the focus is on the ‘doer’ or ‘the subject’, the postman, because the doer is actively performing the action. When we change the sentence into passive voice, the focus shifts from ‘the doer’ of the action to ‘the receiver of the action, that is, ‘a single letter’.