Correct option is (a) Lignite
Easiest explanation: Lignite is the secondary stage in the formation of coal. It is brown in color and hence known as brown coal. It is used as a low grade fuel especially for domestic use and also as a boiler fuel. It is the intermediate stage between bituminous coal and peat. It has a calorific value of about 12500 kJ/kg. It consists of 66% C; 5% H2; 20% O2; 1% N2; 1% S and 3.5% ash and other incombustible.