Answer:
D. According to a health expert, the whole world will be safer if everyone can effectively fight certain diseases.
Step-by-step explanation:
According to a health expert, the whole world will be safer if everyone can effectively fight certain diseases would be the most relevant piece of evidence for the research question "Does the world have a responsibility to make sure poor countries have enough medicines for their people?" as it directly addresses the question of whether global health is a shared responsibility and whether it is in everyone's best interest to ensure access to medicines for all.