Haemoglobin is a protein found in red blood cells that is responsible for transporting oxygen from the lungs to the tissues and carrying carbon dioxide back to the lungs. Affinity refers to the strength of the bond between a molecule and haemoglobin. In this case, the question is asking which molecule has the highest affinity for haemoglobin.

Option 1, oxygen, is one of the main molecules that haemoglobin binds to. It has a relatively high affinity for haemoglobin, allowing for efficient oxygen transport.

Option 2, carbon dioxide, is also transported by haemoglobin, but it has a lower affinity compared to oxygen. This lower affinity promotes the release of carbon dioxide from the blood into the lungs for exhaling.

Option 3, carbon monoxide, has the highest affinity for haemoglobin compared to the other options. Carbon monoxide binds to haemoglobin much more strongly than oxygen, leading to a decrease in oxygen-carrying capacity and potentially causing carbon monoxide poisoning.

Option 4, nitrogen, does not have a significant affinity for haemoglobin and is not involved in its normal function.

Hence, the correct answer is option 3, carbon monoxide. Alert - correct answer should be option 1, oxygen.