Which construction material is least likely to be classified as combustible?

Steel is classified as non-combustible, making it the least likely material among the options provided to be classified as combustible. Unlike wood, plastic, and paper, which can easily ignite and contribute to fire development, steel does not burn and can withstand high temperatures without igniting. This characteristic makes steel a preferred choice in construction, particularly in fire-resistive applications and structural frameworks where fire safety is a critical concern.

In contrast, wood, plastic, and paper are all organic materials that possess flammable properties and can support combustion. Wood can catch fire relatively easily, and while plastic is also combustible and can release toxic fumes when burned, paper is highly susceptible to ignition. Understanding the differences between these materials is essential in assessing fire safety and compliance with building codes.