Non-conventional sources of energy:
Non-conventional sources of energy are those sources that are not commonly used or are not traditionally considered as the primary sources of energy. These sources are often renewable and have a lesser impact on the environment compared to conventional sources of energy.

Explanation:
The correct answer is option 'B' because atomic energy, biogas, and solar energy are non-conventional sources of energy. Let's understand each of these energy sources in detail:

Atomic Energy:
Atomic energy, also known as nuclear energy, is generated through nuclear reactions. It involves the process of splitting the nucleus of an atom (nuclear fission) or combining the nuclei of atoms (nuclear fusion). Nuclear power plants use nuclear fission to generate electricity. It is a non-conventional source of energy because it is not widely used and has certain associated risks and concerns such as radioactive waste disposal and the potential for accidents.

Biogas:
Biogas is produced through the decomposition of organic matter in the absence of oxygen. It is primarily composed of methane gas, which can be used as a fuel for heating, cooking, and electricity generation. Biogas can be produced from various sources such as agricultural waste, animal manure, sewage sludge, and food waste. It is considered a non-conventional source of energy as it is a renewable resource and helps in waste management.

Solar Energy:
Solar energy is obtained from the sun's radiation and can be converted into various forms of energy such as heat and electricity. Solar power can be harnessed using photovoltaic (PV) cells, solar thermal systems, and concentrated solar power (CSP) plants. It is a non-conventional source of energy as it is abundant, renewable, and does not produce greenhouse gas emissions during operation.

Conclusion:
In conclusion, atomic energy, biogas, and solar energy are non-conventional sources of energy. They offer alternatives to conventional sources such as coal, petroleum, and natural gas, and have the potential to reduce environmental impact and promote sustainability.