Which class in the UN classification system pertains to flammable solids?

The classification of flammable solids falls under Class 4 in the UN classification system. This class specifically encompasses materials that are solid and can ignite through friction, heat, or contact with moisture. The ability of these materials to catch fire easily poses significant safety risks, which is why they are categorized for specific handling and storage requirements.

In addition to flammable solids, Class 4 also includes other categories such as classifying substances that are spontaneously combustible or those that release flammable gases when in contact with water. Understanding these classifications is essential for proper handling, transport, and storage in order to prevent accidents and ensure fire safety.

Therefore, identifying Class 4 as the correct answer ensures an understanding of how such materials should be managed to minimize the risks associated with fire hazards.