What defines a "drug"?

The definition of a "drug" hinges on its ability to alter normal bodily function, which is precisely why the correct choice is focused on this characteristic. Drugs can interact with the body in various ways, such as affecting mood, perception, behavior, and overall physiological processes. This definition encompasses a broad spectrum of substances, including legal medications, illegal drugs, and even certain over-the-counter products. The disruptive or altering effects of drugs on the human body are fundamental to understanding their potential impact on health and behavior.

The other options focus on narrow aspects of substances that do not capture the full essence of what a drug is. For instance, while hallucinations may occur with some drugs, not all drugs produce such effects, and this definition excludes many important medications. Similarly, stating that a drug is simply a chemical made in a factory limits the definition to synthetic compounds and ignores naturally occurring substances that also function as drugs. Lastly, while many drugs are indeed prescribed by medical professionals, this option does not account for over-the-counter drugs or illicit substances, which are also classified as drugs. Thus, the most comprehensive definition is the one that highlights a substance's ability to alter normal bodily function.