Which statement describes Class 4 flammable solids?

• A. They ignite spontaneously or when in contact with water
• B. They burn only when heated
• C. They are always solids
• D. They require oxygen from air to burn

Answer: (A)

Class 4 flammable solids are defined by how they pose a fire hazard rather than just being solids. They include materials that ignite on their own in air (pyrophoric solids) and substances that release flammable gas when they come into contact with water. Because of these two real-world behaviors, the statement that best describes them is that they ignite spontaneously or when in contact with water.

Saying they burn only when heated would miss the spontaneous ignition and water-activated cases. Saying they are always solids restates the category name but doesn’t capture the hazard behavior. Saying they require oxygen from air to burn is true for many combustibles but doesn’t address the specific ways Class 4 materials can ignite without an external heat source or produce flammable gas upon contact with water.