The correct option is D
Differentiated cells.Mammalian erythrocytes also called as red blood cells are formed in bone marrow from multipotential hemocytoblast and further converted to erythroblast (normoblast). The nucleus and mitochondria of erythroblast disappear and it is converted to mature erythrocyte with haemoglobin. So RBC's are differentiated cells which lack a nucleus.
so, the correct answer is 'Differentiated cells.'