Final answer:
Flumazenil is the medication a nurse should anticipate administering to a client who has overdosed on diazepam, as it is an antidote to benzodiazepine overdoses.
Step-by-step explanation:
A nurse caring for a client in the emergency department following a diazepam overdose should anticipate administering Flumazenil to the client. Flumazenil is an antidote to benzodiazepine overdose. While both naloxone and naltrexone are opioid antagonists used to treat opioid overdoses, they would not be effective in the case of a diazepam overdose, which is a benzodiazepine. Atropine is used to treat certain types of nerve agent and pesticide poisonings, as well as slow heart rate and to reduce salivation and airway secretions during surgery, not benzodiazepine overdoses. Therefore, the correct medication to administer would be Flumazenil (option B).