Final answer:
The most effective method to prevent backflow is a valve, which is present in both veins and arteries to control the direction of blood flow.
Step-by-step explanation:
The most effective and dependable method of preventing backflow is a valve. Valves are present in both veins and arteries to prevent the backflow of blood. In veins, they ensure that blood flows only towards the heart, while in arteries, they ensure that blood flows only away from the heart.