Quinine is an anti-malarial drug. It is also an anti-arrhythmic drug. It is effective against almost all varieties of the malarial parasite. The product is obtained from the bark of the Cinchona tree. Thus, quinine adds to the economic and medicinal value to the Cinchona plant. However, nowadays many cases of quinine resistant parasites have emerged.
So, the correct answer is 'Bark of Cinchona stem'.