Hint: RBCs are the cellular component of blood, millions of which give the blood its characteristic colour in the circulation of vertebrates, and average life in humans is 100–120 days.
> Option A is incorrect. The capillaries in the walls of the alveoli are blood vessels. Blood flows through the capillaries, reaches your pulmonary artery, and departs through your pulmonary vein. While in the capillaries, the blood releases carbon dioxide into the alveoli through the capillary wall and absorbs oxygen from the alveoli 's air.
> Option B is correct. The mature human red blood cell is small, round, and biconcave; in profile, it appears stupid bell-shaped. It is coated with a lipid-protein membrane, lacks a nucleus, and contains hemoglobin — a red iron-rich protein that binds oxygen. The cell is versatile and assumes the form of a bell as it traverses incredibly narrow blood vessels. The shape of the biconcave raises the surface area of the cell relative to a flat disc of the same dimension. The greater surface area makes the passage of gases into and out of the red blood cell smoother.
> Option C is incorrect. The absorption of oxygen into the blood through the walls of the lung alveoli happens simply by diffusion or an active physiological mechanism and the form of the biconcave helps in the ability of a red blood cell to move and carry oxygen and organs and tissues by tiny blood vessels.
> Option D is incorrect. The protein hemoglobin is a molecule that contains almost all of the oxygen in the blood. It consists of four subunits, each comprising a heme group plus a globin chain.
Hence, The correct answer is option (B).
Note: In the bone marrow, the red blood cell grows in many stages: it becomes an erythroblast (normoblast) from a hemocytoblast, a multipotential cell in the mesenchyme; The erythroblast gradually fills with hemoglobin over two to five days of development, and its nucleus and mitochondria (particles in the cytoplasm that provide energy for the cell) disappear.