In which situation must you NOT pass a vehicle on the right?

Answer: B — When the vehicle ahead is making a right turn

Passing a vehicle on the right is generally prohibited when the vehicle ahead is making a right turn. This is because the vehicle may have slowed down or come to a stop to execute the turn, and passing on the right in this scenario can create a dangerous situation for both drivers. It could lead to a collision if the driver making the turn does not see the passing vehicle or misjudges the distance. This rule is in place to ensure safety on the road, as it allows for traffic to flow smoothly while also preventing accidents that could occur from unexpected maneuvers. The other situations mentioned do not necessarily require you to refrain from passing on the right in the same way.