C is the answer.Premise: Over 80 percent of the people who test-drive a Zenith car end up buying one.
Conclusion: So be warned: you should not test-drive a Zenith unless you are prepared to buy one.
We are to interpret the advertisement to mean that the high percentage of people who buy zenith car after test driving it do so because the quality of the car is unusually high; based on this, we are to find the answer choice which weakens the argument.
We can analyze this argument as a cause and effect argument. In this case, the cause is: A test drive of the Zenith car, while the effect is: purchase the zenith car.
To weaken a cause and effect argument, we need to reverse the cause and effect relationship. What about if people only test drive the car after they have already made up their minds to purchase the car and not that they decide to purchase the car after the test drive? The argument will be weakened since it is not that people decide to buy the car after test driving but rather people test drive the car after making up their mind to buy it.
This is exactly what option C states. Hence C is the right answer.
A) Test-drives of Zenith cars are, according to Zenith sales personnel, generally more extensive than a drive around the block and encounter varied driving conditions. Incorrect as it somehow strengthens the argument rather than weaken it.
(B) Usually dealers have enough Zenith models in stock that prospective purchasers are able to test-drive the exact model that they are considering for purchase. This answer choice neither weakens nor strengthens the argument.
(D) Almost 90 percent of the people who purchase a car do not do so on the day they take a first test-drive but do so after another test-drive. This does not weaken the argument above. It might even be seen as a strengthener because if the first test drive develops an interest in a buyer to request for a second test drive in the future, then this option strengthens the argument rather than weaken it.
(E) In some Zenith cars, a minor part has broken within the first year, and Zenith dealers have issued notices to owners that the dealers will replace the part with a redesigned one at no cost to owners. Irrelevant to the argument. The argument is not about the after-sales service provided to customers but rather about the high likelihood of those that test-drive the car to buy the car after the test drive.