Final answer:
The systemic veins are responsible for returning deoxygenated blood back to the right atrium of the heart, completing systemic circulation. option d.
Step-by-step explanation:
The systemic veins function by returning deoxygenated blood back to the heart. After blood has distributed oxygen to the body's organs and tissues through the capillaries, it becomes oxygen-poor. The blood is then collected by venules that merge into larger veins, ultimately leading to the superior and inferior venae cavae. These major veins deliver the deoxygenated blood to the right atrium of the heart. From the right atrium, the blood is pumped into the right ventricle and then sent to the lungs for oxygenation in the pulmonary circuit. This process is essential for the continuous circulation of blood, providing organs and tissues with oxygen and removing carbon dioxide.