Which of the following refers to how a drug affects the body?

The correct choice focuses on pharmacodynamics, which is the branch of pharmacology that studies how a drug affects biological systems. It includes understanding the mechanisms of action of drugs, the biochemical and physiological effects they produce in the body, and how these effects relate to their therapeutic use. Essentially, pharmacodynamics looks at the relationship between drug concentration and effect, including the drug's therapeutic and adverse effects.

The other choices relate to different aspects of pharmacology. Pharmacokinetics, for example, deals with the absorption, distribution, metabolism, and excretion of drugs—essentially how the body affects the drug rather than how the drug affects the body. Pharmacovigilance focuses on the monitoring of drug safety and the reporting of adverse effects following a drug's release. Lastly, pharmacotherapeutics encompasses the clinical use of drugs to treat disease, including both pharmacodynamics and pharmacokinetics, but it is broader and not solely focused on how the drug impacts the body. Therefore, the emphasis on drug effects in pharmacodynamics makes it the correct answer.