Final answer:
The rest of the blood from venous drainage returns to the right atrium via the vena cava, which includes the superior and inferior venae cavae, as well as the coronary sinus for the heart's blood supply.
Step-by-step explanation:
The correct answer to the question of how the rest of the blood from venous drainage returns to the right atrium is c) Via the vena cava. Blood exiting the systemic capillaries, lower in oxygen concentration, will unite to form larger veins, specifically the superior vena cava and inferior vena cava. These two major systemic veins are responsible for returning deoxygenated blood to the right atrium after it has circulated through the body. Additionally, the coronary sinus drains the blood supply of the heart muscle directly into the right atrium. This deoxygenated blood is then pumped into the right ventricle, which pumps it to the lungs via the pulmonary arteries for gas exchange in the pulmonary circulation.