Pass only at a time.
a. one vehicle
b. two vehicles
c. three vehicles
d. four vehicles

A. one vehicle

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.

answer; ( blue box );