It should be B, A response asking for more clarification.
It wouldn't be A. That does not answer the question at all.
It should be B. This is so they can talk about which medication, why, etc.
It wouldn't be C. Why would the pharmacist say "Oh!"? It doesn't answer the question.
It wouldn't be D. Why would the pharmacist say "no."? They wouldn't.
So yes, your answer is B! :)