Final answer:
The appropriate action by the nurse is to discard the unused medication in the sharps container.
Step-by-step explanation:
Based on the given scenario, the nurse should choose option D, which is to discard the unused medication in the sharps container. It is not appropriate to return the unused portion to the pharmacy as it could potentially be contaminated. Having another nurse witness the disposal of the excess medication is also unnecessary. Placing the syringe with the unused portion in a locked medication drawer is not recommended because it can create confusion and increase the risk of medication errors.