Coal is mainly used as a fuel for power generation. Coal is burned and the resulting heat is used to convert water into steam that drives the turbine. 

There are four major types of coal: anthracite, bitumen, lignite, and peat. 

The classification depends on the form and amount of carbon produced by the charcoal and the amount of heat energy the charcoal provides.
Anthracite is the best and contains the most carbon. The carbon content is about 92-98%. 

Lignite is a brownish-black color with a carbon content of about 60 to 70 percent, often up to 75 percent water content, and 6 to 19 percent ash.
Therefore, the proportion of coal contained in lignite is 60-70%.

Hence, the correct option is D.