I'm going with E...
This one is interesting because at first glance there seems to be more than one answer that may be correct. After closer inspection, I believe that only (E) can be considered correct.
P1: Larry has purchased a device that the manufacturer claims will reduce the fuel consumption in his car.
P2: After a month has passed, Larry determines that his mileage currently rests at 17 miles per gallon.
P3: Steve, owns the exact same make and model car, and has calculated his mileage at 23 miles per gallon.
P4: Steve's car does not have the device that Larry purchased one month ago.
C1: The manufacturer of the device's claim is not true.
Which of the following statements would cause Larry's conclusion to be the weakest?
A) "Though Larry has the same make and model of car as Steve, Larry's car is 15 years older" Incorrect. This answer is irrelevant. It is our assumption that an older car gets worse gas mileage. There is no information in the passage to support this assumption. As far as this passage is concerned, age has nothing to do with mileage. Therefore, this answer does not weaken the conclusion in any way.
B) "Larry was driving in the city, whereas Steve drives the highway" Incorrect. Same flaw as in (A). This answer is irrelevant. It is our assumption that highway driving gets better gas mileage than city driving. There is no information in the passage to support this assumption. As far as this passage is concerned, where you drive has nothing to do with mileage. This answer does not weaken the conclusion in any way.
C) "Larry purchases a lower grade of gasoline than Steve" Incorrect. Again, same flaw as in (A) and (B). This answer is irrelevant. It is our assumption that a lower grade of gasoline gets worse gas mileage than a higher grade. There is no information in the passage to support this assumption. As far as this passage is concerned, fuel grade has nothing to do with mileage. This answer does not weaken the conclusion in any way.
D) "Steve lied, he actually only gets 15 miles per gallon" Incorrect. This one is a little harder to rule out. In the passage, Larry and Steve have the same car and Larry installs a device in his car that is supposed to reduce fuel consumption. After installing the device and calculating his mileage Larry concludes that the manufacturer's claim is false because his car gets worse gas mileage that Steve's car.
In my opinion Larry's reasoning is flawed. In this situation he shouldn't compare his car to Steve's car. He needs to compare HIS car's mileage before installing the device to HIS car's mileage after installing the device in order to come to any meaningful conclusion. Even though Steve and Larry have the same car there can be some variance in fuel consumption (no two cars get exactly the same mileage).
Let's say that (D) is true and Steve did lie about his mileage. Does this weaken the conclusion? No. Just because Steve's mileage is 15 doesn't necessarily mean that the device works, and consequently, Larry's conclusion is weakened. What if Larry's car got 17 miles per gallon before the device. Then the device had no effect on his car's mileage and the conclusion holds.
In my opinion, Larry is comparing apples to oranges (or maybe apples to slightly different apples). Just like in (A), (B), and (C), Steve's mileage is irrelevant to this conclusion. The conclusion is best weakened by pointing out the flaw in the argument. This is what I believe answer (E) does.
E) "Before buying the device, Larry had never before calculated the mileage of his car." Correct. As mentioned above, this answer points to the flaw in Larry's reasoning. If he doesn't know his car's mileage before the device how can he come to any meaningful conclusion about the device's performance.
Larry's reasoning is analogous to this: Imagine you are running a marathon. You hire a personal trainer that claims he can improve your performance. After you run the race you take note of your finish time. You notice that your twin brother ran a faster time than you. Therefore you conclude that the personal trainer's claim is false. Does this make any sense? Of course not. To arrive at a meaningful conclusion you need to compare YOUR performance before the personal trainer to YOUR performance after the personal trainer.
Since we know that Larry did not calculate his mileage before the device we can effectively throw out his conclusion. In my opinion, this answer choice makes Larry's conclusion meaningless.
