The correct answer is option 1, Iron. Haemoglobin is a complex protein found in red blood cells that is responsible for carrying oxygen from the lungs to the rest of the body. It consists of four subunits, each of which contains a heme group. The heme group contains an iron atom, which is crucial for the binding and transport of oxygen. Iron allows the haemoglobin molecule to easily bind to oxygen molecules in the lungs and then release them in tissues that need oxygen. The iron atom in haemoglobin can also bind to carbon dioxide and carry it away from tissues, helping in the removal of waste products. Therefore, iron is a necessary component of haemoglobin and plays a fundamental role in oxygen transport within the body.