Haemoglobin, the protein responsible for carrying oxygen in our blood, contains the metal iron. This iron is essential for the binding and transport of oxygen molecules.

Option 1: Copper is not found in the structure of haemoglobin. It is found in other proteins such as copper-containing enzymes.

Option 2: Molybdenum is not found in the structure of haemoglobin. It is found in other proteins such as molybdenum-containing enzymes.

Option 3: Iron is the correct answer. It is an essential component of haemoglobin, allowing it to bind oxygen and transport it throughout the body.

Option 4: Magnesium is not found in the structure of haemoglobin. It is found in other proteins and serves various roles in the body.

Therefore, the correct answer is option 3 - Iron.