The correct answer is D. Haemoglobin.
Haemoglobin is a protein found in red blood cells that carries oxygen from the lungs to the rest of the body. It is made up of four subunits, each of which contains a heme group. The heme group is a complex of iron and protoporphyrin IX, which is a ring structure with four nitrogen atoms. The iron atom in the heme group can bind to oxygen, and when it does, the haemoglobin molecule changes shape. This change in shape allows the haemoglobin molecule to release oxygen to the tissues.
Keratin is a protein found in hair, nails, and skin. It is a tough, fibrous protein that helps to protect these structures.
Myoglobin is a protein found in muscle cells. It stores oxygen and releases it to the muscles when they need it.
Collagen is a protein found in connective tissue. It is a strong, flexible protein that helps to hold the body together.