Final answer:
Flares can be categorized as a 'reactive flammable material' based on their chemical property of flammability, which is their capacity to catch fire and burn.
Step-by-step explanation:
The student's question 'Flares are a ________.' can be answered using the concept of chemical properties, in which flammability is a key characteristic to consider. Flares are designed to produce a bright light for signaling, illumination, or for other purposes, and they undergo a combustion reaction to do so. Therefore, flares fall under the category of a reactive flammable material, as they are specifically formulated to react and burn. In the context of chemical properties, flammability refers to a substance's ability to catch fire and burn in the presence of an ignition source, which is distinct from physical properties like melting point, boiling point, and density.
In exercises asking about chemical properties, option a. flammability is usually correct, while melting point, boiling point, and density are considered physical properties. In the case of flares, their ability to combust, or their flammability, is the chemical property that defines their intended use.