The correct option is B cytoplasm of RBCs
During the maturation of RBCs, haemoglobin accumulates in the cytoplasm of RBCs.
Mammalian RBCs are devoid of organelles like nucleus, mitochondria and endoplasmic reticulum. They are biconcave in shape. This is done so that maximum haemoglobin can be transported by RBCs. So, haemoglobin cannot accumulate in these organelles inside an RBC.