Among the given options, only NH3 (ammonia) is alkaline. When ammonia gas dissolves in water, it produces ammonium hydroxide (NH4OH), which is an alkaline solution. The other gases, NO2, CO2, and N2 are not alkaline gases. NO2 is an acidic gas, CO2 is a neutral gas, and N2 is also a neutral gas since it does not have any tendency to either lose or gain electrons.