Which hazards are linked to Class 4 flammable solids?

Class 4 flammable solids are known for their potential to ignite easily and can present significant risks in various environments. The correct choice highlights that some of these solids can spontaneously ignite, meaning they are capable of igniting without an external ignition source when exposed to air or heat. Additionally, being water-reactive indicates that these substances may react with water, potentially releasing flammable gases or causing other dangerous reactions. This combination makes Class 4 flammable solids particularly hazardous in scenarios where they may come into contact with moisture or high temperatures.

The other choices do not accurately capture the nature of Class 4 flammable solids. They do not remain harmless in a liquid state, nor are they typically non-toxic. In fact, many flammable solids can be quite dangerous and may cause more severe reactions than minor irritations. Additionally, the idea that they can only combust under extreme pressure is misleading, as they can ignite under more common and less extreme conditions. Therefore, the emphasis on spontaneous ignition and water reactivity aligns well with the characteristics that define the hazards of Class 4 flammable solids.