Acid rain is a type of rain that has a high level of acidity, typically caused by the release of pollutants into the atmosphere, such as sulfur dioxide (SO2) and nitrogen oxides (NOx). These pollutants react with the water, oxygen, and other substances in the atmosphere to form various acidic compounds, such as sulfuric acid (H2SO4) and nitric acid (HNO3), which then fall to the ground as acid rain.

Carbon monoxide (CO) is not a significant contributor to acid rain because it does not react with water in the atmosphere to form acidic compounds. Nitrogen oxide (NOx), on the other hand, is a major contributor to acid rain because it reacts with water, oxygen, and other substances in the atmosphere to form nitric acid (HNO3), which is a strong acid. Ozone (O3) is also a contributor to acid rain because it reacts with water and other substances in the atmosphere to form a variety of acidic compounds. However, ozone is not a primary pollutant that leads to acid rain.

Sulfur dioxide (SO2) is a major contributor to acid rain because it reacts with water, oxygen, and other substances in the atmosphere to form sulfuric acid (H2SO4), which is a strong acid. When sulfur dioxide is released into the atmosphere, it can travel long distances before it is converted into sulfuric acid, which can then fall to the ground as acid rain.

Therefore, the correct answer to the question is 2 and 4 only, as nitrogen oxide and sulfur dioxide are the two pollutants that are major contributors to acid rain.