Which condition could lead a driver to be placed out of service during a roadside inspection?

Answer: B — Keeping a false log

Keeping a false log is a serious violation that can lead a driver to be placed out of service during a roadside inspection. Logbooks are critical for tracking a driver's hours of service and ensuring compliance with regulations designed to prevent fatigue-related accidents. When a driver falsifies their log, it not only indicates a disregard for federal regulations but also poses a significant risk to safety on the road. This violation suggests potential risks associated with driver fatigue, making it a priority for law enforcement during inspections. In contrast, the other conditions mentioned—driving an unregistered vehicle, having outdated registration, and driving an underweight vehicle—may lead to penalties or fines but do not typically rise to the level of an immediate risk to safety that would warrant being placed out of service. Falsifying logs indicates an intent to conceal violations related to hours of service and can lead to dangerous driving conditions, hence the stricter repercussions.