The correct answer is (a). 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 an iron atom. The iron atom binds to oxygen, and when the haemoglobin reaches the tissues, it releases the oxygen so that it can be used by the cells.
(b) is incorrect because haemoglobin does not destroy microbes. This is the job of the immune system, which uses white blood cells to fight off infection.
(c) is incorrect because haemoglobin does not prevent blood deficiency. This is the job of the bone marrow, which produces new red blood cells.
(d) is incorrect because haemoglobin does not make iron useful. Iron is an essential mineral that is used by the body to produce haemoglobin. However, haemoglobin does not make iron useful; it is simply a protein that carries iron around the body.