Final answer:
The trade discount is calculated by multiplying the list price by the single discount rate, which is option A.
Step-by-step explanation:
The formula that gives the trade discount when the list price and single discount rate are given is "list price times single discount rate." To calculate the trade discount, you take the list price of an item and multiply it by the discount rate (expressed as a decimal). For example, if the list price of an item is $100 and the single discount rate is 10%, you would convert 10% to 0.10 (in decimal form) and multiply it by the list price to find the trade discount: $100 × 0.10 = $10. Hence the trade discount amount will be $10.
Following this calculation, you can determine the net price (the price after discount) by subtracting the trade discount from the list price: $100 - $10 = $90.