Correct answer: D
The argument assumes that congestion is caused by the type or size of vehicles using the roads, and therefore giving priority to smaller vehicles will reduce congestion.
Choice D seriously weakens this assumption. If the main reason for traffic congestion is actually poor adherence to traffic laws (such as blocking intersections, illegal parking, or ignoring signals), then changing vehicle priority based on size will not address the real cause of the problem. Even if smaller vehicles are prioritized, congestion would continue because the underlying issue is driver behavior, not vehicle size.
The other options do not weaken the proposal:
A supports the idea that prioritizing vehicle size can reduce congestion.
B and E describe the problem getting worse, not why the solution would fail.
C suggests that most vehicles are already small, but it does not explain why prioritizing them would be ineffective.
Therefore, D casts the most doubt on the effectiveness of the proposed solution.