Select the correct answer.Walter sees a sign on a store window announcing a 25 percent discount on a minimum purchase of $500. What type of discount is the store offering?membership discountcontingent discountstraight discountbulk-purchase discount

I believe this would be a contingent discount. Contingent discounts are granted to a buyer when they satisfy certain requirements. In this case, the requirement is to have a minimum purchase of $500. Thus, that would make it a contingent discount.
The other's are incorrect.
A membership discount requires you to have a membership. 
A straight discount is basically present value of future cash flow without compounding. (Business Dictionary Definition)
A bulk purchase discount is a financial incentive for regular shoppers or businesses that buy things in bulk.