Answer: The right answer is the A) La receta.
Explanation: Just to elaborate a little on the answer, it can be added that to say that the pharmacist is reading the pills ("pastillas"), the drugs ("medicamentos") or the antibiotic ("antibiótico") does not make much sense. The most suitable word is "receta," which can be translated as prescription—this word also means "recipe," and the pharmacist could be in fact also reading a recipe to prepare a specific medicine.