In what situation is it illegal to pass another vehicle?

Answer: B — At a stop sign or red light

It is illegal to pass another vehicle at a stop sign or red light because these traffic control devices require vehicles to come to a complete stop. Passing in this scenario could create dangerous situations by potentially interfering with crossing pedestrians or other vehicles that are following the rules. The law aims to maintain order and safety at intersections, where the risk of accidents is heightened due to the convergence of moving vehicles and pedestrians. In contrast, while passing is generally discouraged on a one-lane road, it might be permissible under certain circumstances if safe to do so, particularly if there's a clear opportunity to pass on a multi-lane road. Similarly, passing when going downhill is not illegal per se, but it can be dangerous due to the increased speed and loss of control that may occur. During high traffic conditions, while passing is fraught with risk, it is not outright illegal unless specific signs dictate so. Thus, the most straightforward and universally applicable rule regarding illegal passing situations is at a stop sign or red light, where compliance is essential for safety.