DNA profiling techniques of blood require DNA sample and we know that DNA is present in the nucleus of cells.
Option A is incorrect as serum is the light yellow liquid left after blood clotting and does not contain any blood cells.
Option B is incorrect as erythrocytes are enucleated cells and hence lack DNA.
Option D is incorrect as significantly platelets are small fragments of megakaryocytes and do not have the nucleus.
Option C is correct as leucocytes are large cells with nucleus which in turn house DNA that can be used as the sample for DNA profiling of the blood stains.