The correct answer is Erythrocyte.

- Erythrocytes, commonly known as red blood cells, are unique among the blood cells because when they reach full maturity, they do not contain a nucleus.
- This anatomical feature allows more space within the cell to accommodate hemoglobin molecules, which are essential for the transport of oxygen throughout the body.
- The absence of a nucleus also contributes to the biconcave shape of erythrocytes, enhancing their flexibility and ability to pass through narrow capillaries.

- In contrast, other blood cells such as lymphocytes, monocytes, and neutrophils are types of white blood cells (leukocytes) and retain their nuclei throughout their lifespan.
- The presence of a nucleus in these cells is important for their function in immune response, as it allows them to produce proteins and respond to pathogens effectively.

Key points:
- Erythrocytes are anucleated when fully mature.
- Absence of nucleus allows greater hemoglobin content and flexibility.
- White blood cells retain their nuclei for immune functions.

Reference: Robbins Basic Pathology, 10th Edition, Chapter 2: Cell Injury, Cell Death, and Adaptations, p. 45