Which of the following describes a flame?

A flame is best described as a product of combustion that appears as a visible luminous body of burning gas. This phenomenon occurs when fuel reacts with an oxidizer, typically oxygen, producing heat and light. The flames we see are primarily made up of hot gases that are glowing due to the energy released from this chemical reaction.

Flames can vary in color, size, and temperature, depending on the type of fuel being burned and the combustion conditions. For example, the presence of certain chemicals can influence the color of the flame, while the temperature can affect its brightness and size.

This understanding distinguishes flame from the other options provided. A solid body undergoing combustion is more about the material burning rather than the flame itself. A gaseous substance generated by a fire may refer to emissions or byproducts of combustion but does not capture the defining characteristics of what constitutes a flame. A reaction resulting in a liquid state does not describe a flame; it may refer to a different phase of matter that can occur during certain combustion processes but does not define the flame itself. Thus, describing a flame as a visible luminous body of burning gas encapsulates its essence in the context of combustion.