Which of the following error is an error of principle?

<p><strong>a. Wages paid to R has been debited to his account </strong></p> <p><strong>b. The total of purchase journal has not been posted to purchase account </strong></p> <p><strong>c. Sales to X Rs.200, posted to his account as Rs.2000</strong></p> <p><strong>d. Furniture purchased on credit from N posted to machinery account </strong></p>