The proportion of carbon in coal increases as the rank of coal enhances due to the coalification process which concentrates carbon by reducing water, oxygen, and hydrogen content.
As the rank of coal increases, the proportion of carbon in it typically increases. This is because the process of coalification, which involves the physical and chemical transformation of plant material into coal, leads to an increase in carbon content and a decrease in volatile components as the rank progresses from peat through lignite and bituminous to anthracite.
With reference to the question provided, as the rank of coal increases, the proportion of carbon in it increases. This happens due to the loss of water, oxygen, and hydrogen, which leaves behind a higher concentration of carbon in higher-ranked coals such as anthracite.
So, higher-rank coals contain a greater proportion of carbon, which is an essential feature in identifying the quality and usability of coal.