Final answer:
Naloxone is the medication to administer for a client with heroin toxicity as it is an opiate antagonist that rapidly reverses the effects of opioid overdose.
Step-by-step explanation:
The appropriate medication to administer to a client with heroin toxicity who exhibits symptoms such as being unresponsive, having pinpoint pupils, and a significantly reduced respiratory rate, is Naloxone. Naloxone is an opiate antagonist that counteracts the life-threatening effects of opiate overdose, including the severe respiratory depression caused by heroin. It works by rapidly displacing opiates from the receptor sites in the brain, which can reverse the toxic effects of the overdose. Unlike other options listed in the question, Naloxone is designed specifically for opioid toxicity and has no effect on individuals not experiencing opioid overdose, making it the safest and most effective choice in this emergency situation.