What is defined as any substance that alters the way the body functions, excluding food, oxygen, and water?

The correct answer is drug, as it directly refers to any substance that, when introduced to the body, can change its normal physiological functions. Drugs can have various effects, such as altering mood, perception, or physical function, and they include a wide range of substances, from prescription medications to illicit drugs and over-the-counter products.

When defining drugs, it's important to note that the options concerning supplements, medicine, and chemicals do not capture the essence of the definition provided in the question. Supplements typically refer to products taken to enhance health or nutrition and are not primarily substances that alter body functions in a medical or pharmacological sense. Medicine is a specific type of drug used for treatment, prevention, or diagnosis of conditions, making it a subset of drugs rather than a broad definition. Chemicals encompass a wide array of substances, only some of which can be classified as drugs, as they don’t exclusively alter bodily functions in the same intended manner as drugs do, which are specifically designed or identified for such purposes.

Understanding this distinction clarifies why drug is the most appropriate term for the definition provided in the question.