Concept:Trial balance disagreement occurs only due to one-sided errors, where one side of an account is incorrect while the other remains unchanged.
Explanation:A wrong addition in the sales account affects only the debit or credit side of that single account, leaving the double entry incomplete. Therefore, the total debits and credits in the Trial Balance will not match.
The other given errors do not disturb Trial Balance agreement because they maintain complete double entry. Crediting a purchase to the sales account is an error of principle or commission with both sides recorded. Failure to enter sales in the books is an error of omission. Posting the purchase of a van to the debit side of the purchases account is also a complete double entry, though posted to the wrong account.
Only one-sided errors, like wrong addition, affect the Trial Balance agreement.
Answer:A. wrong addition in the sales account