The blood-brain barrier occurs when the physical structures of the brain prevent blood from intermingling with the extracellular fluid of brain cells. this separation makes it difficult and dangerous to deliver drugs to specific regions of the brain. therefore, further research on how the blood-brain barrier works is needed to allow more effective delivery of drugs. what potential ethical issues may inhibit such research?

Potential ethical issues of this research would begin with obtaining research subjects. Human brains and animal brains work differently, therefore, research scientists would need to find willing  human participants. Then, the issues would continue if there were deaths because of the research. Also, there's always the potential to further injure a participant.

I don't know the context of this question, but those are some ethical issues I can foresee. 

Your answer is going to be c.