Answer:
A. one vehicle
Step-by-step explanation:
When you are driving, you shoud only pass one vehicle at a time because this movement generates the risk of having an accident. So, you should only try to do it if it is absolutely necessary and one vehicle at a time in places where it is allowed and without passing the speed limit. However, there isn't a law that forbids to pass more than one vehicle but it is a matter of avoiding unnecessary risks.