The correct answer is B. No nucleus.
Fully mature human RBCs do not have a nucleus. This is because the nucleus is no longer necessary for the function of the cell. The main function of RBCs is to carry oxygen to the tissues. The nucleus is not required for this function. In fact, the presence of a nucleus would actually make it more difficult for RBCs to carry oxygen. This is because the nucleus would take up space that could be used for hemoglobin, which is the protein that carries oxygen.
The nucleus is also a site of protein synthesis. However, RBCs do not need to synthesize new proteins, as they are already fully formed when they are released from the bone marrow.
Therefore, it is advantageous for RBCs to lose their nucleus as they mature. This allows them to carry more oxygen and to function more efficiently.
Option A is incorrect because fully mature human RBCs do not have a nucleus.
Option C is incorrect because the nucleus is not present in fully mature human RBCs.
Option D is incorrect because the nucleus is not present in fully mature human RBCs.