Concept:Red blood cells (erythrocytes) in mammals are the only cells among the options that lack a nucleus.Explanation:A mature erythrocyte has no nucleus, giving it more space to carry haemoglobin.This also makes the cell flexible so it can pass through narrow blood vessels.Osteocytes are bone cells and they contain a nucleus.Neurones are nerve cells and they contain a nucleus.Leucocytes are white blood cells and they contain a nucleus.Therefore, the absence of a nucleus is a feature of the erythrocyte.Answer:C. erythrocyte