Transboundary pollution, also known as acid rain, is caused by option 4: nitrogen oxide and sulphur dioxide. Nitrogen oxide is produced from the burning of fossil fuels in vehicles and power plants, while sulphur dioxide is released from industrial processes like the burning of coal and oil. When these pollutants are released into the atmosphere, they can react with moisture in the air to form nitric acid and sulphuric acid. These acids then fall back to the earth as acid rain.

Options 1, 2, and 3 are incorrect. Carbon monoxide is a harmful gas produced from incomplete combustion, but it is not directly responsible for acid rain. While carbon dioxide is a greenhouse gas that contributes to climate change, it is not a primary cause of acid rain. Hydrocarbons, which are organic compounds consisting of hydrogen and carbon, do not directly contribute to acid rain.

Therefore, the correct answer is option 4: nitrogen oxide and sulphur dioxide.