The Most Severe Restraint Imposed by the Constitution and the Courts

The most severe restraint imposed by the Constitution and the courts on police behavior in investigations, and one that can end up letting persons who are almost certainly guilty go free, is:

A) the Fourth Amendment, which requires police to issue a warrant before making a search.
B) the Miranda rule, under which arrested persons must be made aware of their rights before questioning.
C) the probable cause rule, which requires law enforcement to have a reason to believe that a crime may have been committed before detaining someone, or making a search.
D) the exclusionary rule, which prohibits evidence obtained during an illegal search from being introduced in a trial.