In a situation where a vehicle must stop at a railroad crossing, when is it permissible to proceed?

The correct choice is grounded in ensuring safety at railroad crossings, which are critical areas where vehicles and trains might interact. Waiting until a train has passed and it's confirmed safe to proceed is the most responsible approach. This method guarantees that there are no hidden or unexpected trains that could be approaching from either direction, maintaining vigilance for both visible signals and any obstructions.

In contrast, proceeding right after the signal turns off or when warning lights stop flashing can be misleading. While it may seem logical to assume these signals indicate it's safe, they can sometimes be less reliable if the area is not properly monitored or in disrepair. Additionally, railway operations may not always provide ample warning, meaning that a train could still be in the vicinity despite the absence of visible signals or lights.

Finally, assuming it's safe to move immediately when no train is visible does not account for potential blind spots, curves in the tracks, or scenarios where a train might approach unexpectedly. This underscores the importance of erring on the side of caution in matters of safety, particularly at railroad crossings.