Which materials are included in Class 4 flammable solids?

Class 4 flammable solids include materials that are capable of igniting and burning easily in standard conditions. The correct answer identifies various materials that fall into this category. Cobalt powder and potassium are well-known examples of flammable solids.

Cobalt powder can ignite spontaneously when it is finely divided, especially in the presence of moisture. Potassium is a highly reactive alkali metal that ignites in air and reacts violently with water, making it a significant fire hazard.

The other options may include materials that have different properties. For instance, silicone is generally considered non-flammable, while aluminum and zinc can form reactive dusts but do not fit the typical classification of flammable solids in the same manner as cobalt and potassium. Lead and copper do not exhibit flammable properties and are more commonly regarded as non-flammable metals, further delineating their exclusion from Class 4 flammable solids. Thus, the characteristics of cobalt powder and potassium meet the criteria for this specific classification, validating the correctness of the selected answer.