Final answer:
You should not drive faster than the posted maximum speed limit on a highway, which in this case is 120 km/h, regardless of other traffic speeds. Option number a is correct.
Step-by-step explanation:
The question pertains to road traffic laws and the appropriate driving speed on a highway. When driving on a highway with a speed limit of 120 km/h, despite observing that most traffic moves at 125 km/h, you should adhere to the maximum posted speed limit, which is 120 km/h. This is not only the law but also a matter of safety.
Exceeding the speed limit can lead to traffic fines or more severe consequences in the event of an accident.
It is important to note that while some drivers may exceed speed limits, this does not make it legal or safe for others to do the same. Always abide by the posted speed limits, which are there to ensure the safety of all road users.