Yes Immature RBC's are Known as reticulocytes.They are devoid oF nucleus Just like Mature RBC.

The Correct Answer and Explanation:
The correct answer to the question is option 'D', which states that immature red blood cells (RBCs) of mammals have no nucleus.

Explanation:
In mammals, the production of red blood cells, also known as erythropoiesis, occurs in the bone marrow. The process involves the differentiation and maturation of hematopoietic stem cells into mature red blood cells.

During the early stages of erythropoiesis, the developing red blood cells, known as erythroblasts, undergo several changes. Initially, erythroblasts have a nucleus, but as they mature, the nucleus becomes smaller and eventually gets expelled from the cell. This allows more space for hemoglobin, the protein responsible for oxygen transport, to be synthesized and packed into the RBC.

Key Points:
- Immature red blood cells in mammals are called erythroblasts.
- Erythroblasts initially have a nucleus.
- As erythroblasts mature, the nucleus becomes smaller.
- Eventually, the nucleus is expelled from the cell, and the RBC becomes enucleated.
- Mature red blood cells in mammals do not have a nucleus.

Why RBCs lose their nucleus:
The loss of the nucleus in red blood cells is a crucial adaptation for their function. The absence of a nucleus allows red blood cells to have a biconcave shape, which increases their surface area-to-volume ratio. This shape is important for efficient oxygen and carbon dioxide exchange in the capillaries.

Additionally, the absence of a nucleus in mature red blood cells provides more space for hemoglobin, enabling the RBCs to carry more oxygen molecules. Without a nucleus, red blood cells are also more flexible and can easily squeeze through narrow capillaries.

Conclusion:
In conclusion, immature red blood cells of mammals have a nucleus, but as they mature, they lose their nucleus. The absence of a nucleus in mature red blood cells allows for their unique shape and increased oxygen-carrying capacity.