In which scenario is it illegal to pass another vehicle?

Passing on a double solid line is illegal because those markings indicate that overtaking another vehicle is prohibited in that area. Double solid lines are specifically designed to enhance safety by preventing drivers from attempting to pass when it may be unsafe to do so, such as on winding roads or in areas with limited visibility. Violating this rule can lead to serious accidents, as it often indicates a lack of space or an unsafe environment to complete a passing maneuver. Therefore, adherence to these markings is crucial for maintaining road safety and preventing collisions.

The other options involve scenarios where passing may be permissible under certain conditions, assuming it is safe to do so. However, passing on a double solid line is explicitly forbidden, hence making it the definitive legal restriction in this context.