Final answer:
The best indication that morphine has been effective is when the client rates their pain at a 3.
Step-by-step explanation:
The best indication that morphine has been effective in relieving pain is when the client rates their pain at a 3. This indicates that the pain has significantly decreased since the administration of the morphine. While vitals within normal limits, not requesting additional medication, and resting comfortably with eyes closed are all positive signs, they do not directly measure the effectiveness of morphine in pain relief. The client's self-report of pain level provides the most accurate indication of the drug's effectiveness.