Which term describes chemicals that act on living systems at the molecular level?

The correct answer is "drug" because drugs are defined as chemicals that have a biological effect on living organisms by interacting with specific molecular targets such as proteins, enzymes, or receptors. This interaction can alter physiological functions or biochemical processes, resulting in therapeutic effects, side effects, or toxicity. Drugs can be naturally occurring or synthetically manufactured and are integral in the treatment and management of various medical conditions.

The other terms hold different meanings within pharmacology. Synergism refers to the phenomenon where two substances work together to produce a greater effect than the sum of their individual effects, but it does not directly define the function of a chemical as a drug. Toxicology is the study of the adverse effects of chemicals, including drugs, on living organisms, focusing primarily on toxicity rather than the therapeutic action. Brand name refers to the commercial name given to a drug by its manufacturer, which is distinct from the drug's chemical identity or its action at the molecular level.