Which of the following best defines a flammable substance?

A flammable substance is best defined as one that is capable of burning with a flame. This definition is important because it indicates that the material can sustain combustion and produce a readily visible flame, which is a significant characteristic when assessing fire hazards. While the presence of smoke (producing smoke indicates combustion, but not all smoking substances are inherently flammable, as some may smolder without creating a flame) or the ability to ignite without a flame (which describes ignitability but does not necessarily imply the material will burn visibly or sustain combustion) are relevant in fire investigations, they do not capture the essential quality of being classified as flammable. Additionally, burning without oxygen describes a process outside the normal requirements of combustion, as fire typically requires an oxidizing agent, usually oxygen, to sustain a flame. Therefore, the focus on producing a flame provides a clearer and more practical understanding of flammability in the context of fire safety and investigation.