Which one is not responsible for the formation of acid rain?

Explanation

Acid rain is primarily caused by the dissolution of specific oxides in atmospheric moisture, leading to acidic compounds. Carbon monoxide (CO) is not capable of forming an acid when it reacts with water, thus it does not contribute to acid rain. In contrast, carbon dioxide (CO2) can form carbonic acid, sulphur dioxide (SO2) forms sulphuric acid, and nitrogen dioxide (NO2) forms nitric acid, all of which contribute to acid rain. Therefore, CO is the only option that does not lead to acid rain formation.