In which part of the body blood gets purified (Becomes oxygenated) ?

Heart (Right atrium) receives deoxygenated blood from different parts of body and then from right ventricle through pulmonary artery deoxygenated blood is transferred to lungs. In the lungs blood gets purified where deoxygenated blood is converted into oxygenated blood and then through pulmonary vein oxygenated blood is again transferred back to heart (left atrium) and then through aorta oxygenated blood is supplied to all parts of body.

Since, purification of blood i.e. conversion of deoxygenated blood to oxygenated blood takes place in lungs. Therefore, correct option is B. Lungs

Following picture depicts the complete process: