The correct answer is option 3, zinc. Insulin is a hormone that helps regulate the level of glucose in the blood. In its natural form, insulin is a protein that is produced by the pancreas in the human body. However, when insulin is manufactured for medical use, it may contain small amounts of zinc. Zinc is added during the production process to help stabilize the insulin molecule and prolong its shelf life. Zinc acts as a cofactor, assisting in the formation of the insulin hexamer, which is important for the storage and release of insulin within the body. Therefore, while the main component of insulin is a protein, it can also contain trace amounts of zinc as a necessary component for its stability.