Which of the following describes white phosphorus?

White phosphorus is primarily known as a marker for artillery. It has unique properties that allow it to create a visible smoke screen, which enhances targeting and coordination on the battlefield. When ignited, white phosphorus produces a dense, white smoke that can effectively signal troop positions or direct strikes, making it a valuable tool in military operations.

The other options do not accurately describe white phosphorus. It is not categorized as a high explosive, as its primary function is not to detonate and cause destruction like traditional explosives. While it has been used in some conflicts and can cause severe burns, it is not classified as a chemical weapon in the traditional sense, since its use primarily serves a purpose beyond chemical warfare. Lastly, white phosphorus is highly flammable, which is contrary to the description of it being a non-flammable substance. Thus, the characteristics and typical applications of white phosphorus firmly align with its role as a marker for artillery.