Concept:An error that changes only one side of an account will make the trial balance totals disagree.
Explanation:The trial balance agrees only when total debits equal total credits.
Undercasting means that one side of an account is totalled to a figure that is lower than its correct total.
So only the debit side or only the credit side is affected, causing the trial balance totals to differ.
Errors of commission, compensating errors, and errors of principle do not affect the agreement of the trial balance totals.
Therefore, undercasting is the error that affects the agreement of the trial balance.
Answer:B. undercasting