In which of the following situations is it illegal to pass another vehicle?

Passing another vehicle is illegal on hills and curves primarily due to safety concerns. When driving on a hill or around a curve, visibility is often limited, making it difficult to see oncoming traffic or other potential hazards. If a driver attempts to pass in these situations, they may not have enough time or space to return safely to their lane, which could lead to collisions. Therefore, traffic laws prohibit passing in these areas to help ensure the safety of all roadway users.

In contrast, while passing at intersections, in residential areas, and near gas stations may also present risks, these locations may not have blanket prohibitions against passing. For example, at intersections, the legality of passing depends on specific circumstances, such as the presence of signals or stop lines, and in residential areas, passing might be allowed if it's safe to do so. However, the rule against passing on hills and curves is stricter due to the inherent dangers posed by reduced visibility.