Which of the following is a non-renewable source of energy?

"$A$ biogas plant solves the fuel as well as environmental problems". Discuss.

Describe the steps involved in obtaining biogas and explain what is meant by anaerobic decomposition.

Which type of process is currently used in nuclear electricity generation?

Aditya suggests his family to install a solar water heater at their residence. But some of the family members were in favour of installing an electric geyser.
$(a)$ Who,according to you,is taking a correct decision? Mention the value exhibited by Aditya.
$(b)$ Also,give reasons (at least $2$) for your answer.

What constitutes a source of energy?