Arrange the following in the increasing order of their degree of Coalification…

Concept

Coalification is the progressive geochemical alteration of buried plant remains into coal under steadily rising temperature and pressure over geological time. Its degree is what geologists call the rank of a coal: the further the transformation has advanced, the higher the carbon content and calorific value, and the lower the moisture and volatile-matter content. Formal schemes grade the lowest ranks chiefly by calorific value and the higher ranks by fixed carbon, but both indicators track the same underlying carbon-enrichment scale. Ranking these materials therefore means placing them along that single scale.

Applying the concept to the five given materials

Placed on that scale from the least transformed to the most transformed:

1. Peat — the unconsolidated precursor stage of coal formation; carbon content only about 50–60% on a dry, mineral-matter-free basis, with the highest moisture of the series.

2. Lignite — brown coal; carbon content about 60–70%, soft, crumbly and still very moist.

3. Sub-bituminous coal — carbon content about 70–76%, distinctly drier and more compact than lignite.

4. Bituminous coal — carbon content about 76–86%; the dense, dark, widely mined household and coking grade.

5. Anthracite — carbon content about 86–95%; the hardest, most lustrous and highest-rank coal, with the least volatile matter.

Cross-check

Rank can be verified against a second, independent indicator — moisture — which must fall as carbon rises:

The carbon column rises monotonically while the moisture column falls monotonically down the same series, so both independent measures of rank agree on one and the same ordering.

Reading the labels off that verified series, the increasing order of the degree of coalification is D, E, C, A, B.