The correct answer is: A. Credit notes and delivery challans.
A credit note is a document issued by a seller to a buyer to acknowledge the return of goods or the cancellation of a sale. A delivery challan is a document issued by a supplier to a customer to acknowledge the delivery of goods.
In case of sales return, the auditor should check the credit notes and delivery challans to ensure that the sales return is genuine and that the goods have been returned to the supplier. The auditor should also check the purchase invoices and goods received notes to ensure that the goods were originally sold to the customer.
Option B is incorrect because the auditor should not only check whether cash has been repaid to the client, but also whether the goods have been returned to the supplier.
Option C is incorrect because the auditor should not only check the purchase invoices, but also the goods received notes.
Option D is incorrect because the auditor should not only check the credit notes, but also the delivery challans.