What remains of the ancient plant matter in lignite coal primarily composed of?

Lignite coal, often referred to as brown coal, is primarily composed of carbon along with various minerals. During the formation of lignite, the original plant matter undergoes a process of diagenesis, which transforms the organic material into a carbon-rich substance.

Carbon content is a significant factor because it defines lignite as a precursor to higher ranks of coal, such as sub-bituminous and bituminous coal. The mineral content may consist of clays, quartz, and other trace minerals which contribute to the rock's overall composition but in smaller proportions compared to carbon.

The other options focus on components that do not characterize lignite as accurately. For instance, silica and ash are often products found in higher-grade coals or secondary aspects of coal but aren't primary constituents of lignite. Water and gases are present in lignite but do not represent the material's main composition. Iron and aluminum, while present in certain forms of rock and minerals, do not constitute a major part of lignite's composition. Therefore, the focus on carbon and minerals in lignite emphasizes the essential nature of this type of coal in the geological category it belongs to.