Erythrocytes, also known as red blood cells (RBCs), function to transport oxygen in the blood. They do so with the help of haemoglobin, a respiratory pigment. The shape and structure of erythrocytes are ideal for this function. They lack a nucleus and few other organelles so that maximum space in the cell can be used to accommodate haemoglobin molecules. Leucocytes are involved in immune response while thrombocytes are involved in blood clotting. Blood plasma does not contribute to O2 transport. Thus, option B is correct.