The accident _______ if the motorist had obeyed the speed limit sign.

The phrase "The accident _______ if the motorist had obeyed the speed limit sign" requires a form that reflects a hypothetical situation in the past. Option A, "Never would have happened," uses the correct conditional perfect form to convey that the accident was a possibility that could have been avoided if a different action had been taken.

This construction indicates that the accident did happen because the condition (the motorist obeying the speed limit) was not met. It suggests that in an alternate reality where the motorist adhered to the speed limit, the accident would not have occurred.

In contrast, other options do not fit the required grammatical structure for a past unreal conditional statement. For instance, the second option implies a present certainty rather than addressing a past condition. The third option states a simple past that does not suggest the hypothetical nature of the situation, and the fourth option lacks proper word order, making it grammatically incorrect and unable to convey the intended meaning of possibility related to the past scenario.