The accident _____ if the motorist had obeyed the speed limit sign.
Correct Answer: B
Rationale: The correct answer is 'never would have happened.' This choice correctly expresses the hypothetical outcome that did not occur due to the motorist obeying the speed limit sign. The phrase 'never would have happened' is the most suitable option in this context as it indicates a past event that was prevented by following a certain condition. Choice A ('could have happened') is incorrect as it does not convey the same level of certainty and definiteness as 'never would have happened.' Choice C ('wouldn't have occurred') is close but lacks the emphasis on the event not taking place at all. Choice D ('could happen') is in the present tense and does not fit the context of a past hypothetical situation.