The government has implemented a new policy that requires all newly

The argument from the car manufacturers is that the new policy requiring emissions tests on newly built cars will not significantly decrease overall pollution levels because older vehicles contribute more to pollution. To strengthen this argument, we need to find a choice that supports the claim that the new policy will have a minimal impact on overall pollution levels due to the presence of older vehicles.

A) New cars make up a small percentage of the total number of cars on the road.

This statement directly supports the manufacturers' argument. If new cars are indeed a small percentage of all cars, then even if they are made to be less polluting, the overall impact on pollution levels would be minimal due to the larger number of older, more polluting cars on the road.
B) The new emissions test is more expensive than previous tests, leading to higher costs for consumers.

While this statement talks about the impact of the policy on consumers' costs, it does not address the relative contribution to pollution by new versus old vehicles, so it does not strengthen the manufacturers' argument about pollution levels.
C) Many car manufacturers had already been voluntarily implementing similar emissions standards before the policy was mandated.

This choice suggests that the new policy may not have a significant additional impact on emissions because manufacturers were already adhering to similar standards. However, it does not directly address the pollution contribution of older vehicles versus new ones.
D) Older vehicles are, on average, less fuel-efficient than newly built cars.

This statement could be interpreted as weakening the argument because it implies that older vehicles could be more polluting due to their lower fuel efficiency. Hence, it does not strengthen the manufacturers' position.
E) The government is also considering a policy that would provide tax incentives for car owners to replace their old cars with new, low-emission vehicles.

This choice suggests a future action that could reduce pollution from older vehicles, but it does not directly strengthen the argument about the current situation and the impact of the new policy on overall pollution levels.
Therefore, the correct answer is A, as it most directly supports the car manufacturers' argument by indicating that new cars are a minor part of the problem when it comes to overall pollution.