The correct answer is option 3, Carbon monoxide. Haemoglobin is a protein in red blood cells that is responsible for carrying oxygen throughout the body. It has the highest affinity for carbon monoxide, meaning that it binds more tightly to carbon monoxide than to other gases. This is a dangerous situation because when carbon monoxide binds to haemoglobin, it prevents oxygen from binding properly, leading to decreased oxygen delivery to tissues and organs. Carbon monoxide poisoning can occur when individuals are exposed to high levels of carbon monoxide, such as from car exhaust or faulty heating systems. It is important to note that while haemoglobin also binds to other gases such as oxygen, carbon dioxide, and nitrogen, its affinity for carbon monoxide is the highest.