The correct answer is B) flammable liquids. Foam-type portable fire extinguishers are most effective at extinguishing fires involving flammable liquids, such as gasoline, oil, or paint. The foam blankets the surface of the liquid, preventing oxygen from reaching the fire and cooling the fuel, which helps to extinguish the flames. The other answer choices are not the best applications for a foam-type extinguisher. A) combustible metallic solids require a dry powder or carbon dioxide extinguisher, C) solid materials are better suited for water or dry chemical extinguishers, and D) electrical equipment fires are most effectively addressed with a carbon dioxide or dry powder extinguisher to avoid potential damage from the extinguishing agent.