Final answer:
The observation that indicates the medication has been effective is when the patient rates her current pain lower after the intervention.
Step-by-step explanation:
The observation that indicates that a patient's pain medication has been effective in managing pain that she rated a 6 out of 10 before the intervention is option b: The patient rates her current pain as 3 out of 10 on the pain rating scale. When the patient rates her current pain lower after receiving the medication, it suggests that the medication has successfully reduced her pain level.
Options a, c, and d may not necessarily indicate the effectiveness of the pain medication. Quietly reading a magazine, telling her family she is feeling better, and sleeping with a reduced respiratory rate are general behaviors that can occur even without the medication being effective in managing pain.