The correct answer is option 2, Bark. Cinchona is a tree that belongs to the Rubiaceae family. It is mainly known for its bark, which contains several alkaloids, including quinine. Quinine is a drug that is obtained from the bark of the cinchona tree and is commonly used to treat and prevent malaria. The bark is harvested from mature trees, and the outer layers are removed to extract the medicinal compounds. The pericarp refers to the outer covering of a fruit, so option 1 is incorrect. The endosperm is the nutrient-rich part of a seed, and option 3 is unrelated to cinchona. The leaf of the cinchona tree does contain some alkaloids but in lower concentrations compared to the bark, making option 4 less relevant.