Concept:Embers are small, glowing pieces left behind when a fire is nearly extinguished.Explanation:The word "embers" refers to fragments of burning coal, wood, or other fuel that continue to glow after the main fire has died down.Among the given choices, "coal" matches this meaning most closely because embers are often described as glowing coals.The other options — crystal, flakes, and frost — do not relate to fire or burning material.Answer:D. Coal