When is it illegal to pass another vehicle?

Answer: B — When approaching a hill or curve where visibility is limited

Passing another vehicle is illegal when approaching a hill or curve where visibility is limited because it poses significant safety risks. In these situations, drivers cannot see oncoming traffic or the road conditions ahead, which increases the likelihood of collisions. Safety regulations are designed to prevent accidents, and this particular rule aims to protect both the driver attempting to pass and oncoming vehicles. It is crucial to maintain awareness of visibility limits when driving, as these conditions can lead to dangerous scenarios if overtaking is attempted. While passing may be restricted in other scenarios, the specific limitation associated with hills or curves relates directly to the compromised visibility that can result in severe accidents.