Which type of extinguisher is effective against flammable metals?

The correct choice is the Class D fire extinguisher, which is specifically designed for use on flammable metal fires, such as those involving magnesium, titanium, sodium, and potassium. These types of metals can ignite and burn at extremely high temperatures and require specialized extinguishing agents that do not react with the burning metal. Class D extinguishers typically contain materials like dry powder agents that smother the fire or absorb heat without reacting with the burning metal, thereby effectively controlling and extinguishing flames.

Class A, B, and C extinguishers, while effective against ordinary combustibles, flammable liquids, and electrical fires respectively, are not suitable for flammable metal fires because they can either be ineffective or could potentially worsen the situation. For instance, water from a Class A extinguisher can react violently with certain flammable metals, exacerbating the fire rather than extinguishing it. Therefore, only a Class D extinguisher is appropriate for dealing with fires involving flammable metals.