The one which is not considered as naturally occurring greenhouse gas is

The correct option is ethane.

To understand why ethane is not considered a naturally occurring greenhouse gas, let us analyze the nature of greenhouse gases and the specific roles of each option listed:

1. Greenhouse Gases (GHGs): Greenhouse gases are gas molecules in the Earth's atmosphere that absorb and emit infrared radiation, trapping heat and warming the planet. The major naturally occurring greenhouse gases in Earth's atmosphere include water vapor ($H_{2}O$), carbon dioxide ($C{O}_2$), methane ($C{H}_4$), nitrous oxide ($N_{2}O$), and ozone ($O_3$).

2. Analysis of the Options:
• Carbon dioxide ($C{O}_2$): This is a primary and highly abundant greenhouse gas released naturally through processes like respiration, volcanic eruptions, and organic decay.
• Methane ($C{H}_4$): A potent greenhouse gas produced naturally by anaerobic decomposition of organic matter, such as in wetlands, and by the digestive tracts of ruminant animals.
• Nitrous oxide ($N_{2}O$): Another significant greenhouse gas produced naturally in soils and oceans through microbial activity in the nitrogen cycle.
• Ethane ($C_2{H}_6$): Ethane is a simple alkane and a trace hydrocarbon found in natural gas. While it is present in small amounts and can undergo oxidation to form carbon monoxide and ozone, it is not classified or targeted as a major, direct naturally occurring greenhouse gas that contributes directly to the planetary greenhouse effect in the same way as carbon dioxide, methane, or nitrous oxide.

Therefore, among the options provided, ethane is the one that is not considered a naturally occurring greenhouse gas.