Step 1: Concept
Pharmaceutical ethics guide the professional conduct of a pharmacist toward patients, prescribers, and the public.
Step 2: Meaning
A pharmacist must maintain professional boundaries and support the prescriber's therapeutic plan.
Step 3: Analysis
Consulting a doctor about errors (A) or availability (B) is professional. However, discussing the "demerits" of a prescribed drug's efficacy with a patient can undermine their confidence in the treatment and the physician.
Step 4: Conclusion
Such a discussion is considered an unethical act for a pharmacist.
Final Answer: (D)