To determine which of the given gases is not a greenhouse gas, we need to understand what qualifies a gas as a greenhouse gas.$\\$Greenhouse gases are those that can trap heat in the atmosphere, contributing to the greenhouse effect.$\\$Common greenhouse gases include:$\\$• Carbon dioxide (CO$_2$)$\\$• Methane (CH$_4$)$\\$• Nitrous oxide (N$_2$O)$\\$• Ozone (O$_3$)$\\$• Water vapor (H$_2$O)$\\$Let's analyze each option:$\\$Option 1: Nitrous oxide (N$_2$O)$\\$• Nitrous oxide is a well-known greenhouse gas.$\\$Option 2: Methane (CH$_4$)$\\$• Methane is a potent greenhouse gas, much more effective than CO$_2$ at trapping heat.$\\$Option 3: Ozone (O$_3$)$\\$• Ozone in the lower atmosphere acts as a greenhouse gas.$\\$Option 4: Sulphur dioxide (SO$_2$)$\\$• Sulphur dioxide is not considered a greenhouse gas. Instead, it can lead to cooling effects by forming aerosols that reflect sunlight.$\\$Therefore, the gas that is not a greenhouse gas is Sulphur dioxide (SO$_2$). This corresponds to Option 4.