For a number 5, I think this is a pretty interesting problem. Here's the breakdown:
Conclusion: Advocates of herbs should always be allowed to prescribe them.
Premises: Many herbs are safe to consume.
There is little firm evidence that herbs have medicinal effect.
Patients prescribed herbs will not be harmed, and might be helped by them.
(C) is a tempting trap choice for many folks, but a trap that can be avoided by focusing on the conclusion of this argument and being aware of the distinction between "medicines" and "herbs"
(A) Yes, this weakens the conclusion. If patients who are prescribed herbal medicines (w/little chance of medicinal effect) tend to neglect effective conventional medicines, it seems that the prescribing of herbal medicines should not, in fact, "always" be allowed.
(B) This does not effect the conclusion one way or another.
C) Has no effect not he conclusion.
(D) Again, no effect on the conclusion, which relies on evidence about the likelihood of harm to a patient, regardless of the "motive" of the "purveyor."
(E) This hardly effects the arguments conclusion, though, if anything, could strengthen it.
The correct answer is (A).