Which characteristic defines materials that are 'Dangerous When Wet'?

Materials classified as 'Dangerous When Wet' are specifically characterized by their potential to become spontaneously flammable or to emit a flammable gas when they come into contact with water. This classification is crucial for ensuring safety in handling these substances, especially in environments where water is present, such as during firefighting or in case of spills.

The dangerous nature of these materials stems from chemical reactions that can occur when they interact with water. For example, certain metals or reactive chemicals may decompose in water to release gases that can ignite or that are themselves flammable. Understanding this property is essential for emergency responders and those working with hazardous materials to mitigate risks effectively.

In contrast to this correct answer, materials that emit a toxic gas upon contact with water do present a hazard but are not classified as 'Dangerous When Wet' unless they also exhibit the reactivity characteristic defined by option C. Similarly, while some substances might dissolve in water, this does not inherently make them dangerous in the way that option C does. Finally, materials that are non-reactive with water do not fall under the 'Dangerous When Wet' classification at all, as they do not pose any specific danger when exposed to water.