The correct answer is option 2, Iron. Haemoglobin is a protein found in red blood cells that is responsible for carrying oxygen throughout the body. It binds to oxygen in the lungs and releases it in the tissues. Iron is a crucial component of haemoglobin, as it forms the heme group that binds to the oxygen molecules. Without iron, haemoglobin would not be able to function properly. Calcium, sodium, and potassium are important electrolytes found in the body, but they are not directly involved in the structure or function of haemoglobin. Thus, the correct element found in haemoglobin is iron.