What would have prevented the accident if the motorist had obeyed the speed limit?

The correct choice is "Never would have happened" because it conveys a conditional scenario in the past that did not occur. This phrase follows the structure typically used for third conditional sentences, which discuss hypothetical situations in the past and their possible outcomes.

In this context, the sentence suggests that if the motorist had obeyed the speed limit, the accident was a possibility that could have been avoided. Using "would have happened" effectively reflects the unreal past condition—indicating that the accident indeed did occur, but under the condition that the driver had followed the speed limit, it would not have happened.

This structure captures both the hypothetical nature of the event and emphasizes the impact of the motorist's action on the outcome of the situation, which is why it fits perfectly within the sentence.