Option (c) is correct
The main ingredient of bio-gas is ethane and it gives a lot of smoke and also produces a lot of residual ash

The above statement holds incorrect as the main gas in biogas is methane which 75% and it doesn’t form smoke or residual ash. Other than methane hydrogen, carbon dioxide and hydrogen sulphide are also liberated which are stored for further use. Option (a) is incorrect
We are encouraged to plant more trees so as to ensure a clean environment and also provide bio-mass fuel

The above statement holds true for biogas generation as more the vegetation more clean air as biogas is made from natural wastages of vegetation or cow dung more smokeless combustion. Option (b) is incorrect
Gobar-gas is produced when crops, vegetable wastes etc., decompose in the absence of oxygen.

This statement is correct as the natural waste is allowed to decompose in absence of oxygen thus forming methane gas. Option (d) is incorrect
Biomass is a renewable source of energy

Biomass is the natural waste material of plants and animals and this is a recurring and inexhaustible phenomenon. So we can say that biomass is a renewable source of energy.