Drug is defined as:

• A. Any substance that will cause a physical or psychological change to a living organism when it enters the body.
• B. Any substance that will cause a physical change to a living organism when it enters the body.
• C. Any substance that will cause a psychedelic change to a living organism when it enters the body.
• D. Any substance that will cause an emotional change to a living organism when it enters the body.

Answer: (A)

The key idea tested is that a drug is any substance that, once inside the body, causes some kind of change in how the body or mind functions. This broad view is important because many substances alter physical processes (like heart rate or digestion) as well as mental states (such as mood, perception, or thinking). The option describing only physical change misses drugs that affect psychology, while the one limiting to psychedelic changes or to emotional changes only excludes many substances that don’t produce those specific effects. So the best choice is the one that covers both physical and psychological changes upon entry into the body, which exactly matches how drugs are understood in this context. For example, caffeine can alter alertness (psychological) and metabolism (physical), while pain relievers primarily affect physical sensation but may also influence mood.