The correct answer is (a) Erythrocytes
Easy explanation: Erythrocytes (Red Blood Cells): Red blood cells have the form of a bi-concave disc with a mean diameter of about 7.5 m and thickness of about 1.7 m. The mean surface area of the cell is about 134mm2. There are about 5.5 million of them in every cubic millimetre of blood in men and nearly 5 million in women.