How are participants assembled for a clinical trial?
a. patients volunteer because they have a specific disease
b. individuals are randomly exposed to a dangerous virus
c. human subjects are given an unnecessary medical treatment.
d. healthy people are infected with a deadly pathogen

I would say the best answer for this question would be A.

Explanation:

I chose this answer because factually, it does check out.  Say, for example there is a clinical trial for a drug to treat a specific disease.  

I can rule out B because obviously these doctors would not purposely infect the volunteer with this disease just to take a shot at curing them.  

I ruled out C because if we were using this trial drug on a completely healthy person... how would we know that the drug can help/cure the disease in mind if the person never had the disease in the first place.  C is simply absurd.  

And finally, D is much like B and so I can rule it out as well for the same reason.  

the answer to your question is slate