Which of the following materials is not considered combustible?

Steel is not considered combustible because it does not ignite, burn, or support combustion under normal conditions. It has a high melting point and can withstand extreme temperatures without catching fire. In contrast, wood, plastic, and glass can all undergo changes when exposed to fire. Wood burns easily, plastic can ignite and release flammable gases, and while glass itself does not burn, it can shatter when exposed to high heat. Thus, steel stands out as a material that is inherently non-combustible, making it the correct answer.