Which class of hazardous materials includes substances that can ignite spontaneously?

The correct response pertains to Class 4 (Flammable solids), as this class specifically encompasses materials that can ignite spontaneously or with minimal external heat. Flammable solids include substances that can easily catch fire through friction, absorption of moisture, or spontaneous combustion upon exposure to air. These materials may pose significant risks, especially in environments where they can create ignition sources.

Class 1 consists of explosives, which require a strong initiation source to detonate, while Class 2 pertains to gases, both flammable and non-flammable, that may not ignite spontaneously. Class 3 encompasses flammable liquids, which ignite at low flash points when exposed to an ignition source, but do not include solids that can ignite spontaneously without an external trigger. Thus, Class 4 is the appropriate classification for materials that can spontaneously ignite.