The pumping of blood occurs from the heart to the tissues. However, before it is sent to the body it must be oxygenated by passing through the lungs.
It does this by passing from the right ventricle through the pulmonary artery to the lungs, where it is oxygenated and returned to the heart through the pulmonary vein. Then passes to the left ventricle which pumps it to the other tissues through the aorta.
The structures in the diagram must be:
a: superior vena cava
b: left atrium
c: pulmonary artery
d: aorta
Because the pulmonary artery sends blood to the lungs, the answer is c (pulmonary artery)