The pulmonary vein is responsible for bringing oxygenated and pure blood into the heart.

The correct answer is option 'A', the pulmonary vein.

The pulmonary vein is responsible for bringing clean, oxygenated blood from the lungs back to the heart. It is an important part of the circulatory system and plays a crucial role in maintaining the body's oxygen supply.

Here is a detailed explanation of the answer:

1. Introduction:
- The circulatory system is a complex network of blood vessels that transports blood throughout the body.
- One of the key components of this system is the pulmonary vein, which carries clean blood from the lungs to the heart.

2. Pulmonary Vein:
- The pulmonary vein is a series of blood vessels that connect the lungs to the heart.
- It consists of four main veins: the right and left superior pulmonary veins, and the right and left inferior pulmonary veins.
- These veins transport oxygenated blood from the lungs back to the heart.

3. Oxygenation Process:
- When we breathe in, the oxygen in the air enters our lungs and diffuses into the alveoli (tiny air sacs).
- The oxygen then binds to red blood cells in the capillaries surrounding the alveoli.
- This oxygen-rich blood is collected by the pulmonary veins and transported back to the heart.

4. Pathway of the Pulmonary Vein:
- The right superior and inferior pulmonary veins collect oxygenated blood from the right lung.
- The left superior and inferior pulmonary veins collect oxygenated blood from the left lung.
- These veins merge together and form a larger vessel called the pulmonary vein.
- The pulmonary vein then enters the left atrium of the heart.

5. Heart's Role:
- The left atrium receives the oxygenated blood from the pulmonary vein.
- When the heart contracts, the oxygenated blood is pumped into the left ventricle.
- From there, the blood is further pumped out to the rest of the body through the aorta, supplying oxygen to the cells.

In conclusion, the vein that brings clean, oxygenated blood from the lungs back to the heart is the pulmonary vein. It is responsible for maintaining the body's oxygen supply and plays a crucial role in the circulatory system.