Concept:Source documents are original business papers that provide evidence for financial transactions.Explanation:The cash book, petty cash book, and general ledger are accounting records, not original documents.A credit note is issued by a seller to a buyer to show a reduction in the amount owed.It is used when goods are returned, an invoice is incorrect, or an overcharge has occurred.Therefore, the only source document listed is the credit note.Answer:D. Credit notes