Final answer:
The Food and Drug Administration's website is the most likely source to provide information about a new medication's side effects and usage.
Step-by-step explanation:
The source that is most likely to provide information about a new medication's side effects and usage is d. The Food and Drug Administration's website. The FDA is responsible for regulating the safety and efficacy of medications in the United States. Their website provides comprehensive and up-to-date information on approved medications, including side effects, usage guidelines, and any warnings or precautions.