What action must you take if you see side-by-side red lights flashing at a railroad crossing?

Answer: B — Stop

When you see side-by-side red lights flashing at a railroad crossing, this is a clear signal that a train is approaching, and you must stop. The red lights indicate that it is not safe to cross the tracks, and stopping ensures your safety as well as the safety of others. Trains cannot stop quickly, and even at high speeds, they require a significant distance to come to a halt. This warning is designed to prevent collisions and accidents at what can be a very dangerous intersection. The red lights serve as a critical reminder to obey the signals and never attempt to cross the tracks until the lights have ceased flashing and it is safe to do so.