Concept:Fossil fuels are carbon-rich energy sources formed from decomposed organic matter over millions of years.Explanation:Lignite is a type of coal, specifically brown coal.Coal is a fossil fuel because it forms from compressed plant remains.Granite and gneiss are igneous and metamorphic rocks.Mica is a silicate mineral, not a fuel.Therefore, among the options, only lignite is a fossil fuel.Answer:C. Lignite