The correct answer is option 3: Carbon monoxide. Carbon monoxide is a gas that readily combines with the haemoglobin of the blood. Haemoglobin is a protein found in red blood cells that carries oxygen throughout the body. When carbon monoxide enters the bloodstream, it competes with oxygen to bind with the haemoglobin. However, carbon monoxide has a much higher affinity for haemoglobin than oxygen, which means it binds to it more easily and strongly.

When carbon monoxide binds to haemoglobin, it forms a compound called carboxyhemoglobin. This process reduces the amount of haemoglobin available for oxygen transport, leading to a condition known as carbon monoxide poisoning. This can result in reduced oxygen supply to the body`s cells and tissues, causing symptoms like headache, dizziness, and shortness of breath. In severe cases, carbon monoxide poisoning can be life-threatening.

Therefore, it is important to minimize exposure to carbon monoxide by ensuring proper ventilation and avoiding the use of devices that produce this gas, such as faulty heating systems or engines running in enclosed spaces.