Final answer:
A credit memorandum is prepared when goods that were sold on credit are returned (option C).
Step-by-step explanation:
When a customer purchases goods on credit, it means that they receive the products immediately but will pay for them at a later date. However, sometimes customers may decide to return these goods for various reasons, such as being dissatisfied or receiving a wrong item.
In such cases, the seller prepares a credit memorandum to document the return and to credit the customer's accounts receivable (amount owed) for the returned goods.