What class includes sodium, cobalt powder, and potassium as examples?

The correct classification for sodium, cobalt powder, and potassium is Class 4 Flammable Solids. This category includes solid materials that can easily ignite and sustain combustion through the materials’ properties or reaction with other substances.

Sodium and potassium are both alkali metals known for their high reactivity, particularly with water, which can produce flammable hydrogen gas. Cobalt powder can also be considered a flammable solid due to its potential to ignite under specific conditions, particularly when finely divided or aerosolized.

Class 4 is focused on substances that, when they are in solid form, pose a risk of fire through various mechanisms, including friction, sparks, or contact with moisture. This class is crucial for safety protocols concerning the handling, storage, and transportation of materials that can ignite easily when exposed to heat or different environmental conditions. The characters of the materials listed align well with those criteria, confirming their placement in Class 4.