The correct answer is option 'D' - Peats.

Peat is a type of organic sedimentary rock that is formed from the accumulation of partially decayed plant material in wet environments such as bogs or swamps. It is considered the lowest grade of coal in terms of calorific value.

Here is a detailed explanation of why peat has the least calorific value compared to other coal varieties:

1. Formation of Peat:
- Peat is formed through the process of peatification, where plant material accumulates in wet and oxygen-poor environments.
- The lack of oxygen slows down the decomposition process, resulting in the partial decay of the plant material and the formation of peat.

2. Composition of Peat:
- Peat consists mainly of partially decomposed plant material, such as mosses, grasses, and tree fibers.
- It has a high moisture content, typically ranging from 75% to 95%.
- The carbon content in peat is relatively low, usually around 50% or less.
- Due to the incomplete decomposition of plant matter, peat contains a significant amount of volatile compounds.

3. Calorific Value:
- Calorific value is a measure of the energy content of a fuel, usually expressed in terms of heat produced per unit mass or volume.
- Peat has the lowest calorific value among all coal varieties due to its high moisture content and low carbon content.
- The high moisture content in peat requires a significant amount of energy to evaporate the water, reducing the available energy for combustion.
- The low carbon content means that peat produces less heat when burned compared to other coal varieties.

4. Comparison with Other Coal Varieties:
- Anthracite coal has the highest calorific value among all coal varieties. It is a hard and highly carbonaceous coal with low moisture content, making it an excellent fuel for heating purposes.
- Bituminous coal has a lower calorific value compared to anthracite but higher than peat. It contains more carbon and less moisture than peat, resulting in a higher energy content.
- Lignite coal has a higher moisture content and lower carbon content than bituminous coal, making it less energy-dense. However, it still has a higher calorific value than peat.

In summary, peat has the least calorific value among all the coal varieties mentioned. Its high moisture content and low carbon content contribute to its lower energy content and make it less suitable as a fuel compared to other coal types.