Concept:White blood cells are part of the immune system and are scientifically called leucocytes.Explanation:Blood contains three main types of cells: red blood cells (erythrocytes), white blood cells (leucocytes), and platelets (thrombocytes).Haemoglobin is not a cell; it is a protein inside red blood cells that carries oxygen.Erythrocytes refers only to red blood cells.Thrombocytes are platelets involved in blood clotting.Thus, white blood cells are correctly termed leucocytes.Answer:Option C. Leucocytes