Final answer:
During active labor, when the client is 8cm dilated, the nurse's first action should be to assess the fetal heart rate.
Step-by-step explanation:
As the client is in active labor and already 8cm dilated, the nurse's first action should be to assess the fetal heart rate (option b). This helps determine the well-being of the baby and any signs of distress. Administering pain medication (option a) can be considered after assessing the fetal heart rate. Encouraging deep breathing exercises (option c) can also be helpful for managing discomfort during labor. Preparing for delivery (option d) may be the next step once the fetal heart rate is assessed and stable.