Although its purpose is laudable, the exclusionary rule, which forbids

The question is not of very high quality.
Nevertheless, the words used by the author indicate that he is assuming 4 of the given 5 options but is not assuming (B).

(A) The constitutional rights of criminal defendants should be protected.
"Although its purpose is laudable... " - he says that the purpose of the rule (not violate defendant’s constitutional rights) is laudable. So he believes that defendant's constitutional rights should be protected.

(B) Most cases in which the exclusionary rule has been invoked have involved purely technical violations of constitutional principles.
"Even when the rights violation was a minor or purely technical one, ..." - just means even in those cases where the violation was technical. It doesn't mean that this is true of MOST cases. Hence the author does not assume this anywhere in the passage.

(C) The number of cases whose outcome has been affected by the exclusionary rule is significant.
"...rule...has unduly hampered law-enforcement efforts" - the author believes that the rule has hampered efforts. If the number of cases were insignificant, we could not have said that the efforts are hampered by it.

(D) Some of the defendants set free under the exclusionary rule have been guilty of serious criminal offenses.
"... defendants who were undoubtedly guilty have been set free, perhaps to steal, rape, or murder again." - notice the use of "again". We can assume that some of these defendants were certainly charged with "stealing", "rape" or "murder", which we can consider serious offenses.

(E) Merely technical violations of the rules concerning evidence should be treated differently from deliberate assaults upon human rights.
"Even when the rights violation was purely technical one, ... rather than on the abrogation of some fundamental liberty,..., the evidence obtained has been considered tainted under this rule" - the author believes that merely technical violations should be treated differently from deliberate assaults.

Answer (B)