Which of the following building materials is considered non-combustible?

Steel is considered a non-combustible material because it does not ignite, burn, or support combustion under normal conditions. Unlike combustible materials, steel maintains its structural integrity and does not produce flames or toxic fumes when exposed to fire. This quality makes steel a preferred choice in construction, particularly for buildings that require enhanced fire resistance.

In contrast, materials like wood and plastic are combustible, meaning they can easily catch fire and contribute to the spread of flames. Drywall, while it has some fire-resistant properties, is primarily made of gypsum board that can still be affected by high temperatures and may contribute to fire's spread under certain conditions. Consequently, in the context of fire safety and building construction, steel stands out as a reliable non-combustible material.