The correct answer is option 4: Stem bark. Quinine, the drug used to treat malaria, is obtained from the stem bark of a plant. The stem bark refers to the outer layer of the plant`s stem. It is in this part of the plant that the active compounds responsible for the antimalarial properties are found. Extracting quinine from the stem bark involves a series of processes including grinding, soaking, and filtering. This allows for the isolation and purification of quinine, which can then be used in the formulation of antimalarial medications. While the other options may have their own medicinal properties, in the case of malaria treatment, it is the stem bark that is specifically used to obtain quinine.