Which of the following is not a major greenhouse gas?

The correct option is ozone.

Greenhouse gases are atmospheric gases that absorb and emit infrared radiation, trapping heat in the Earth's atmosphere and contributing to the greenhouse effect.

Let us analyze the role of each gas listed in the options:
1. Water vapour: This is the most abundant and potent greenhouse gas in Earth's atmosphere, responsible for the majority of the natural greenhouse effect.
2. Carbon dioxide (CO₂): This is the primary greenhouse gas emitted through human activities (such as burning fossil fuels and deforestation) and plays a major role in global climate change.
3. Methane (CH₄): This is an extremely powerful greenhouse gas, with a global warming potential significantly higher than carbon dioxide over a shorter timescale.
4. Ozone (O₃): While ozone in the lower atmosphere (troposphere) acts as a minor greenhouse gas and air pollutant, and stratospheric ozone shields the Earth from harmful ultraviolet radiation, it is generally not classified as a major greenhouse gas in the context of global climate forcing compared to water vapour, carbon dioxide, and methane.

Therefore, among the given choices, ozone is not considered a major greenhouse gas.