Answer:
Prisoners.
Step-by-step explanation:
Department of Health and Human Services (HHS) has specific regulations regarding prisoner research. The IRB or Institutional Review Board has to see and approve that the proposed research falls into permissible category of research. In the Board there has to be a Prisoner Representative which should not be involved in the research.
Throughout history, prisoners used to undergo cruel research and experimentation. Due to this cruelty, many of the modern modern protections arose in response to this, and nowadays prisoners represent one of the highly protected groups of human subjects for research.