What should be placed beside a truck in the event of an emergency as part of vehicle safety compliance?

Answer: B — Flares

In the event of an emergency, placing flares beside a truck is a key safety measure. Flares effectively signal to other drivers that there is an obstruction or hazard ahead, providing them with visual cues to exercise caution. They emit bright light and can be seen from a significant distance, especially in low-light conditions, which is crucial for preventing accidents. While reflective triangles are also a common safety tool for vehicle emergencies, they may not be as immediately visible in all situations compared to the bright light of flares. A safety vest enhances the visibility of a person working near the roadside, but it doesn't directly signal a hazard to oncoming traffic. A fire extinguisher is important for handling vehicle fires but does not serve the purpose of warning other drivers about the presence of the stopped vehicle. Thus, flares are the most effective choice for alerting other motorists in an emergency scenario.