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)
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I think its B as its not assumed unlike other options.

A,D,E are out. Between B/C, I believe B is better answer

I marked Option C as number of cases where final outcome is affected by exclusionary rule is significant cant be the assumption.
VeritasKarishma Mam can you please explain this question... I am unable to deduce how Option B can't be the assumption

I echo the same. And I have chosen the answer only to see it as incorrect. Moreover, This one is something very difficult to eliminite based on the premise given.
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Glad that you thinking matches the one like experts. Please do add your explanations, if you can. This will help ppl like me to judge how other test takers see and think.
Thanks.
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Thanks a tonne for the explanation VeritasKarishma mam -:)

Could you please explain why option (A) is not the answer instead? I agree that the author is in favor of protecting constitutional rights of defendants. However, "criminal defendants" is a bit of a stretch?

"Even when the rights violation was a minor or purely technical one" seems quite opposite to B.

classic example where we cannot eliminate options based on language/criticality of options

I eliminated B, since MOST is mentioned in the option, only to realize language based elimination is not applicable in such [EXCEPT] Questions

Also, I need to understand how "hampered law-enforcement efforts" is guiding us towards Quantity of cases (significant number)

My presumption while reading the question was that the excerpt belongs to a single critical case

KarishmaB ma'am can you clarify the above