The power unit of a placarded vehicle must have a fire extinguisher with a UL rating of:

Answer: B — 10 B:C or more

The requirement for a fire extinguisher in a placarded vehicle is based on the size and potential risk associated with transporting hazardous materials. A fire extinguisher with a UL rating of 10 B:C or more indicates that it is capable of extinguishing fires involving flammable liquids and electrical equipment, which are common hazards in vehicles transporting hazardous materials. This UL rating reflects the fire extinguisher's effectiveness. The "B" classification signifies its suitability for flammable liquids, while the "C" classification indicates it can be used for electrical fires. Having a fire extinguisher with a rating of 10 B:C or higher is essential, as it ensures that the vehicle is equipped to handle potential fire emergencies that could arise during the transportation of hazardous materials. This requirement not only enhances safety for the driver but also for other road users and the environment.