Quinine comes from the bark of the stem — option 3.

The plant. Quinine is an alkaloid extracted from the bark of the Cinchona tree, native to the eastern slopes of the Andes in South America. The bark was long used by the Quechua people, was carried to Europe by Jesuit missionaries in the seventeenth century — hence the name “Jesuits’ bark” — and the pure alkaloid was isolated by the French chemists Pelletier and Caventou in 1820.

Its place in history. Quinine made European settlement of the tropics possible, and it was strategically important enough that the Dutch established plantations in Java and the British in the Nilgiris and in Darjeeling to secure supply. It is also the bitter flavouring in tonic water — the gin and tonic began as a way of taking the daily dose.

How it works, and what replaced it. Quinine kills the malarial parasite Plasmodium in the blood stage. It has largely been superseded by artemisinin-based combination therapy, derived from Artemisia annua, for which Tu Youyou received the Nobel Prize in 2015; quinine is now reserved chiefly for severe or resistant cases. Note that Artemisia yields its drug from the leaves, which is a useful contrast to remember alongside cinchona.

The practical point about bark. Alkaloids are often concentrated in the bark because it is the plant’s defensive tissue — bitter compounds there deter insects and grazing animals, and it is exactly that bitterness which marks the medicinal alkaloids.

Hence, the answer is Stem bark.