Final answer:
When a client's water breaks, the nurse's priority intervention is to assess the fetal heart rate to ensure the baby is not in distress.
Step-by-step explanation:
If a client arrives at the labor and delivery unit and states, "My water just broke," the nurse's priority intervention is b) Assess the fetal heart rate. This assessment is critical to ensure that the fetus is not in distress, particularly after the amniotic sac has ruptured, which can result in changes in fetal pressure and potential complications. After verifying that the baby's heart rate is within a normal range, the nurse can perform other necessary tasks, such as preparing the mother for an ultrasound, inserting an IV catheter, and performing Leopold maneuvers, in no particular order.