Final answer:
A scientist researching undesirable side effects of drugs, when desirable effects are already known, is ensuring the success of the experimental design by comprehensively evaluating the drug's safety and efficacy based on the scientific method. So the correct option is D.
Step-by-step explanation:
If a scientist researches the undesirable side effects of drugs because the desirable effects have already been researched, he is using scientific research in a manner that can be best described as ensuring the success of the experimental design (Option D). This approach is central to conducting a thorough investigation into the efficacy and safety of a pharmaceutical drug. The study of side effects is a critical aspect of the research process, aiming to identify any potential risks associated with a drug's use and ensure that the benefits outweigh these risks. Such research usually involves controlled experiments and statistical analyses to determine the significance of findings.
In a broader sense, this method of research aligns with the principles of the scientific method. The scientific method involves several steps including asking a question, conducting a study, and drawing conclusions. By focusing on side effects, a scientist ensures that the research covers all potential outcomes of the drug's use, thereby contributing to the overall integrity and utility of the research.