Answer: The correct answer is option d.
Step-by-step explanation:
Veins are the blood vessels which carry blood back to the heart. These are less muscular as compared to the arteries. The prevent back flow of the blood with the help of valves.
Valve is a membranous structure which allow the flow of blood in one direction only. They can fold or close in one direction only.
For example, with each pump or contraction when blood flows towards the heart the valves open. However, they close immediately which prevent the back flow.