The correct option is A CO
Carbon monoxide is a colourless, odorless, tasteless, non-irritating and toxic gas formed by incomplete combustion of organic matter. It mainly causes adverse effects by combining with haemoglobin to form 'carboxyhaemoglobin' in the blood. This prevents haemoglobin from carrying oxygen to the tissues, leading to asphyxiation (choking and shortness of breath). In extreme conditions, it can also cause death.