Which of the following defines a combustible liquid?

A combustible liquid is specifically defined by its flash point, which is the lowest temperature at which it can form an ignitable mixture in the air. The characteristic that defines a combustible liquid is having a flash point at or above 100°F and below 200°F. Therefore, the correct answer identifies a liquid with a flash point that meets this definition.

The other options do not accurately reflect the definition of a combustible liquid. A liquid with a flash point below 100°F is classified as a flammable liquid, not a combustible liquid. A liquid that cannot ignite is not classified within combustible or flammable categories, as it does not possess the capability to burn. Lastly, viscosity refers to the thickness or stickiness of a liquid and does not relate to its flammability or combustibility, making it an irrelevant factor in defining combustible liquids.