Which UN Hazard class includes both flammable liquids and solids?

The classification of hazardous materials according to the UN system is based on the specific hazards they pose. Class 4 is designated for flammable solids, spontaneously combustible materials, and materials that are dangerous when wet. This class specifically includes substances that can ignite easily when exposed to a source of ignition, which encompasses a range of solid materials that are flammable in nature.

While Class 1 refers to explosives, Class 2 covers gases (including flammable gases), and Class 3 specifically pertains to flammable liquids, it is Class 4 that uniquely encompasses flammable solids. Thus, the correct answer highlights the inclusion of both types of materials within this class, emphasizing its significance in the transportation and handling of hazardous substances.