Which class of fires involves combustible metals such as sodium and magnesium?

The correct choice is Class D, which is specifically designated for fires that involve combustible metals. This class of fire is unique because it includes materials that ignite and burn in ways that are different from ordinary combustibles, such as wood or paper, as well as flammable liquids. Combustible metals like sodium and magnesium can ignite spontaneously when exposed to moisture or certain conditions, making them particularly hazardous.

Fires classified under Class D require special extinguishing agents, often dry powder agents that can smother the fire or react chemically to cool or mitigate the reaction of the burning metals. Understanding this classification is crucial for safety when dealing with materials that fall into this category, as the use of water or common firefighting methods can exacerbate the situation.

The other classes focus on different fire types, such as organic materials, flammable liquids, and cooking oils, which not only have different combustible properties but also require distinct methods for effective extinguishment.