The correct answer is Naloxone.

- Opioid overdose can lead to respiratory depression, which is a dangerously reduced breathing rate and can be life-threatening.
- This happens because opioids bind to mu-opioid receptors in the central nervous system, suppressing the brain's drive to breathe.

- Naloxone is an opioid antagonist, meaning it competitively binds to opioid receptors without activating them.
- By doing so, naloxone reverses the effects of opioids, particularly respiratory depression.
- It is administered emergently in cases of suspected opioid overdose to restore normal breathing.

Other options are not appropriate for opioid overdose:
- Flumazenil (Option 1) is used to reverse benzodiazepine overdose, not opioid overdose.
- Atropine (Option 3) is an anticholinergic agent used for bradycardia and some poisoning but does not reverse opioid respiratory depression.
- Diazepam (Option 4) is a benzodiazepine and would potentially worsen respiratory depression if opioids are involved.

In summary, naloxone is the first-line treatment for opioid-induced respiratory depression due to its ability to rapidly displace opioids from central receptors, restoring respiratory drive.

Reference: Goodman & Gilman's: The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 19, Opioid Analgesics and Antagonists