The correct answer is option 2, the pulmonary vein. The pulmonary vein is responsible for bringing clean, oxygenated blood from the lungs back to the heart. It is important to note that the other options mentioned are incorrect. The renal vein, option 1, brings blood from the kidneys to the heart, not from the lungs. The vena cava, option 3, actually brings deoxygenated blood from the body back to the heart, not from the lungs. The hepatic vein, option 4, transports blood from the liver to the heart, not from the lungs. Therefore, option 2, the pulmonary vein, is the correct answer as it is the vein that carries clean blood from the lungs into the heart.