Based on the provisions of the Amendment that requires a speedy and public trial, the option prevented is C. Secret trials.
The Sixth Amendment says that when a person is accused of a crime, they are to be granted a trial that is done in public, and is done fast according to the capacity of the judiciary.
A secret trial means that the trial is not public which means that it goes against the 6th Amendment.
In conclusion, option C is correct.
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