Concept:Subsidiary books are books of prime entry where similar transactions are first recorded before being posted to the ledger.Explanation:The main subsidiary books include the sales day book, purchases day book, return inwards day book, return outwards day book, cash book, journal, and petty cash book.A credit note is not a book; it is a source document issued to notify a customer that their account has been credited for returns or overpayment.Therefore, the option that is not a subsidiary book is the credit note.Answer:C. credit note