Non-Greenhouse Gas (GHG) from the given options is Carbon Monoxide (CO).

Explanation:

Greenhouse Gases (GHGs)
- Greenhouse gases are gases that trap heat in the Earth's atmosphere, contributing to the greenhouse effect and global warming.
- The main GHGs include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases.
- These gases are released into the atmosphere through various human activities such as burning fossil fuels, deforestation, and industrial processes.

Identifying the non-GHG:
Out of the given options, Carbon Monoxide (CO) is the non-GHG. Let's briefly discuss each option to understand why CO is the correct answer.

a) Methane (CH4)
- Methane is a potent GHG and has a higher global warming potential compared to carbon dioxide.
- It is released during the production and transport of coal, oil, and natural gas. It also comes from livestock and other agricultural practices.

b) Nitrous Oxide (N2O)
- Nitrous oxide is another GHG with a significant global warming potential.
- It is emitted through agricultural and industrial activities, as well as during the combustion of fossil fuels and solid waste.

c) Sulphur Hexafluoride (SF6)
- Sulphur hexafluoride is a highly potent GHG, known for its use in electrical equipment and as a gaseous insulator.
- It has a very long atmospheric lifetime and a high global warming potential.

d) Carbon Monoxide (CO)
- Carbon monoxide is a colorless, odorless gas that is not considered a GHG.
- It is primarily produced by incomplete combustion of carbon-containing fuels such as gasoline, coal, and wood.
- While it is a pollutant and harmful to human health, it does not have the same capacity to trap heat as the other GHGs.

Conclusion:
In summary, out of the given options, carbon monoxide (CO) is the non-GHG. It is important to identify and understand the different gases and their impact on the environment in order to address climate change and reduce greenhouse gas emissions.