What should a driver do if the trailer starts to sway?

When a trailer begins to sway, or what is often referred to as "snaking," the appropriate action is to reduce speed and avoid sudden changes in steering. This is crucial because a sudden change in speed or abrupt steering adjustments can exacerbate the swaying, making it more difficult to regain control. By reducing speed smoothly, the driver decreases the forces acting on the trailer, which can help to stabilize it. Gradual, measured adjustments to steering will allow the driver to regain control without making the situation worse.

In contrast, increasing speed can worsen the sway, leading to a loss of control. Stopping immediately could also be dangerous, especially if the trailer is swaying on a busy road. Changing lanes quickly can lead to further instability, as it may introduce sudden forces that make the swaying worse. Therefore, option B is the most effective and safe response to a situation where the trailer starts to sway.