A sales day book is a document used by a business to record credit sales transactions made to customers. Credit sales are sales transactions where payment is not received immediately, but rather on a later date, as agreed between the seller and the buyer. The sales day book contains details such as the date of the sale, the name of the customer, the amount of the sale, and the account number of the customer. This information is used to keep track of the total amount of credit sales made during a particular period, and to generate invoices and statements to customers for payment at a later date. Therefore, the correct answer is credit sales.