Concept:A three-column cash book records cash, bank, and cash discount transactions.
Its extra column exists mainly because of cash discount.
Explanation:The three-column cash book is used when a business needs to record cash discount alongside cash and bank transactions.
Cash discount is an allowance given for early or prompt payment of an account.
It has two sides: discount allowed and discount received.
Discount allowed is an expense to the business, while discount received is income to the business.
Therefore, the presence of discount columns is what determines the use of a three-column cash book.
Answer:D. cash discount