What is the minimum fire extinguisher rating required for a placarded vehicle?

The minimum fire extinguisher rating required for a placarded vehicle is 10 B:C. This rating is important because it signifies the extinguisher's effectiveness in combating fires involving flammable liquids and electrical equipment, both of which may be present in a vehicle transporting hazardous materials.

The B rating specifically indicates the extinguisher's capability to deal with fires involving flammable liquids, while the C rating shows it is suitable for electrical fires. The requirement for a 10 B:C rating ensures that the extinguisher has sufficient capacity for the potential fire hazards associated with transporting hazardous materials, thus promoting safety for the vehicle's occupants and others on the road. Higher ratings, like those offered in the other choices, indicate greater capacity but are not the minimum mandated requirement for compliance in this specific context.