Final answer:
Among the given options, the true statement about the structure of a mature, circulating erythrocyte is Erythrocytes can bend and twist to fit through the vessels. So, the option is A.
Step-by-step explanation:
Erythrocytes, commonly known as red blood cells, are biconcave discs that have the flexibility to bend and contort as they pass through narrow capillaries, facilitating gas exchange and transportation. This unique shape and structural adaptability are due to proteins like spectrin, which provide the cytoskeletal support necessary for their durability and flexibility. They are not larger than other cells in the blood but are similar or smaller in size, they are not nucleated, as they extrude their nucleus during maturation, and they are not cell fragments but complete cells.