This weakens the argument by showing that direct-mail advertising might increase sales in stores, which would generally require use of a car, which would add pollutants to the air.

This confirms that without direct-mail advertising, pollutants from in-person sales would actually have occurred. In theory, direct-mail advertisements might have just caused purchases that otherwise wouldn’t have occurred. (B) eliminates this possibility.

The comparative effectiveness of direct-mail ads and magazine ads has no clear impact. We already know that people buy products from home due to direct-mail ads. Even if these are more effective than others, we still don’t know the environmental impact of direct-mail ads.

(D), unlike (B) doesn’t establish that the people who buy products from home would have bought those products without the direct-mail ads. There might be signs those people are more interested in the products, but that doesn’t mean they would buy without the ads.

This doesn’t reveal anything about the environmental impact of direct-mail ads. We still don’t know whether direct-mail ads are causing people to buy from home products that would otherwise be purchased by using a car.