Which of the following is not a book of original entry?

The answer is "Purchases account." A book of original entry is a book where business transactions are first recorded before being transferred to the ledger. The purpose of these books is to provide a chronological record of all transactions. The other options listed are all books of original entry. The sales journal is where all credit sales are recorded, the cash account is where all cash receipts and payments are recorded, and the return inwards journal is where all goods returned by customers are recorded. A "Purchases account" is not a book of original entry, but rather a ledger account used to record all purchases made by the business.