Which of the following best describes an explosive?

The best description of an explosive is a chemical compound, mixture, or device that functions by explosion. This definition encompasses the core characteristics that define explosives: the ability to undergo a rapid chemical reaction that produces a large volume of gas, heat, and pressure. This reaction is what characterizes an explosion.

Explosives can come in various forms, including solid compounds, liquid mixtures, or as part of devices designed to explode. The emphasis on "functions by explosion" highlights the inherent capability of these materials to cause an explosive reaction, which is central to their classification.

Understanding this definition is crucial for anyone studying fire and explosion investigations, as it facilitates recognition of various explosive materials and their potential hazards in both investigative and safety contexts. The other choices do not accurately encapsulate the essential nature and effects of explosives; hence they do not provide the comprehensive definition that option C does.