Deoxygenated blood from organs is carried back to the heart through veins. Veins are blood vessels that transport blood towards the heart. They have thinner walls and larger lumens compared to arteries, which allows them to accommodate larger volumes of blood.

The process of blood circulation involves the movement of blood through a network of blood vessels, including arteries, capillaries, and veins. In this process, oxygenated blood is pumped from the heart to various organs and tissues via arteries. Once the oxygen is delivered to the cells, the blood becomes deoxygenated and collects waste products such as carbon dioxide.

To return to the heart, the deoxygenated blood flows through the smallest blood vessels called capillaries, where exchange of gases and nutrients occurs. After passing through the capillaries, the blood is collected into venules, which then merge to form larger veins. These veins carry the deoxygenated blood back to the heart for it to be pumped to the lungs for oxygenation or to other organs for filtration and distribution.

The main reason why veins carry deoxygenated blood is due to the exchange that occurs in the capillaries. Oxygen is delivered to the cells in the capillaries, and carbon dioxide and other waste products are picked up by the blood. This exchange causes the blood to become deoxygenated and collect waste products. Therefore, the blood that returns to the heart through the veins is deoxygenated.

It is important to note that there is one exception to this rule. The pulmonary veins, which are the only veins that carry oxygenated blood, transport blood from the lungs to the heart. This is because the blood in the pulmonary veins has just been oxygenated in the lungs and is ready to be pumped to the rest of the body.

In summary, veins are the blood vessels that carry deoxygenated blood from the organs back to the heart. The deoxygenated blood is then pumped to the lungs for oxygenation or to other organs for filtration and distribution.

D is correct .