You must avoid _________ while crossing railroad tracks.

The correct answer highlights the importance of maintaining control of your vehicle while crossing railroad tracks. When you stop or shift gears on or very near the tracks, you increase the risk of being stranded on the tracks, which can be extremely dangerous if a train approaches. Additionally, stopping can interfere with the flow of traffic and might make it more difficult for drivers behind you to respond safely.

Maintaining steady motion without unnecessary shifts or stops ensures that you clear the tracks quickly and safely, minimizing the risk of an accident. It's imperative to approach railroad crossings with caution and be prepared to navigate through them without abrupt changes in the vehicle's operation.

While other options indeed relate to safe driving practices around railroad tracks, stopping or shifting gears directly corresponds to the potential risks associated with being on the tracks when a train is approaching, making it the most critical behavior to avoid.