A certain credit-card company awards its customers bonus points for us...
The argument states that customers of a certain credit-card company can use bonus points to purchase brand name merchandise at prices lower than the manufacturers' suggested retail prices. Based on this information, the argument concludes that customers spending their bonus points will spend less than if they purchased the same merchandise in retail stores.
To weaken or challenge this argument, we need to identify an assumption on which the argument depends. In other words, we need to find an answer choice that, if false, would undermine the conclusion of the argument.
Let's examine each answer choice:
(A) The merchandise that can be ordered by mail using the bonus points is not offered at lower prices by other credit-card companies that award bonus points.
This answer choice is not relevant to the argument since it talks about other credit-card companies. The argument only concerns the specific credit-card company mentioned. So, it is not an assumption on which the argument depends.
(B) The bonus points cannot be used by the credit-card customers in the purchase of brand name merchandise that is not available for purchase in retail stores.
This answer choice is also not relevant to the argument. It talks about merchandise that is not available in retail stores, which is outside the scope of the argument. Therefore, it is not an assumption on which the argument depends.
(C) The credit-card company does not require its customers to accumulate a large number of bonus points before becoming eligible to order merchandise at prices lower than the manufacturers' suggested retail price.
This answer choice does not impact the conclusion of the argument. Even if customers need to accumulate a large number of bonus points, it does not affect the claim that they will spend less when using those points compared to retail store prices. Hence, it is not an assumption on which the argument depends.
(D) The amount credit-card customers pay for shipping the merchandise ordered by mail does not increase the amount customers spend to an amount greater than they would spend if they purchased the same merchandise in retail stores.
This answer choice directly addresses the comparison between the amount customers spend using bonus points and the amount they would spend in retail stores. If the shipping cost were to increase the total amount spent on the merchandise ordered by mail, it would weaken the argument's claim that customers spend less. Therefore, this is a crucial assumption on which the argument depends.
(E) The merchandise available to the company's credit-card customers using the bonus points is frequently sold in retail stores at prices that are higher than the manufacturers' suggested retail prices.
This answer choice is not relevant to the argument either. It talks about prices in retail stores, but the argument is specifically comparing the prices customers pay using bonus points with the manufacturers' suggested retail prices. So, it is not an assumption on which the argument depends.
Based on the analysis above, we can conclude that answer choice (D) is the assumption on which the argument depends. If the shipping cost were to increase the total amount spent on the merchandise ordered by mail, it would undermine the claim that customers spend less by using their bonus points.
Therefore, the correct answer is (D).