Concept:An error affects the trial balance total when it makes total debits unequal to total credits.
Explanation:Omission of both sales and purchases entries removes equal debit and credit amounts, so the trial balance still balances.
Posting discount allowed to the debit side of discount allowed account is correct because discount allowed is an expense, so it does not affect the trial balance.
Failure to post both sales and purchases of equal amounts omits equal debit and credit effects, so the trial balance is not affected.
However, omitting one ledger account from the trial balance removes its entire balance from either the debit or credit column, causing the totals to differ.
Answer:C. omission of one account from the list when extracting the ledgers