What structure in veins prevents the backflow of blood?

The correct answer is a valve, as these structures are specifically designed to prevent the backflow of blood in veins. Veins carry blood back to the heart under lower pressure compared to arteries. To assist with this return and to ensure that blood flows in the correct direction, particularly against the force of gravity, veins are equipped with valves. These valves open to allow blood to flow toward the heart and close to prevent blood from flowing backward.

The other structures mentioned do not have this function. For instance, capillaries are tiny blood vessels that facilitate the exchange of gases, nutrients, and waste between blood and tissues, rather than controlling blood flow direction. The aorta, on the other hand, is the body's main artery and carries oxygenated blood away from the heart to the rest of the body. Arteries, similar to the aorta, primarily transport blood away from the heart and do not have valves that prevent backflow. Thus, it is the valves in veins that play the crucial role of ensuring unidirectional blood flow back to the heart.