Which of the following is considered a flammable material?

Paper is considered a flammable material because it can easily ignite and burn when exposed to heat or an open flame. Its composition consists mainly of cellulose fibers, which are organic in nature and can combust relatively quickly under the right conditions. This property makes paper a significant fire hazard in environments where it is not properly managed or stored.

In contrast, concrete is an inorganic material that is non-combustible and does not support combustion. Steel, while it can conduct heat and become very hot, does not catch fire or burn. Similarly, glass is also inorganic, and its structure does not allow for combustion. These materials, while they may be involved in fire scenarios, do not ignite or burn in the way that paper does, which significantly distinguishes flammable materials from non-flammable ones.