Lignite is a type of coal that is characterized as a soft brownish-black sedimentary rock. It is considered the lowest rank of coal due to its relatively low carbon content, typically ranging from 60% to 70%. This type of coal forms from the compaction and dehydration of peat over millions of years, primarily in humid environments.
In contrast, quartzite and granite are types of metamorphic and igneous rocks, respectively, which are not formed from organic materials like coal. Shale, while a sedimentary rock, is composed primarily of clay minerals and does not contain the carbon-rich deposits required to qualify as coal. Therefore, lignite stands out as the correct answer due to its classification and formation processes as a type of coal.