The correct answer is D. Lignite.
Lignite is a type of coal that is less mature than bituminous coal and anthracite. It is also known as brown coal because of its brownish color. Lignite is the lowest rank of coal and has the highest moisture content. It is also the softest type of coal and has the lowest heating value. Lignite is used primarily to generate electricity and to produce coke.
Anthracite is the highest rank of coal and has the lowest moisture content. It is also the hardest type of coal and has the highest heating value. Anthracite is used primarily for heating and cooking.
Bituminous coal is a type of coal that is between lignite and anthracite in rank. It has a higher moisture content than anthracite but a lower moisture content than lignite. Bituminous coal is used primarily for generating electricity and to produce coke.
Coke is a solid fuel that is produced from coal. It is used primarily in the iron and steel industry.