Answer: B. Sales day book

Explanation:
The trade discount is typically recorded in the sales day book when it is offered by the seller to a customer. It is used to show the reduction in the selling price before any payments are made. The sales day book serves as the correct book of prime entry for such transactions, as it records credit sales and discounts offered. Other books, such as the cash account or petty cash book, do not handle these types of records directly, making them incorrect answers.