Answer: Capillaries
Step-by-step explanation: There are two kinds of vessels which carry the blood throughout the body. These are veins and arteries. Now apart from these two, there is a third type which is called as capillaries, which are the connecting vessels between arteries and veins. These are relatively thinner and allow exchange of gases between tissues and the blood. Therefore, arteries and veins are incorrect options.
Heart is the pumping organ, and its function is to pump the blood to every cell of the body, so this can't be correct option.