Final answer:
The highest priority for a nurse caring for a patient who just had a Cesarean section is to monitor the incision site and maintain strict hand-washing practices to prevent infection.
Step-by-step explanation:
The action by the nurse that takes highest priority for a patient with a BMI of 38 who just delivered a baby via Cesarean section is d. Monitoring the incision site and using strict hand-washing technique. The nurse's foremost responsibility is to prevent infection, particularly following major surgery such as a Cesarean section. Administering pain medication is also important, but it does not supersede infection prevention. Assisting the woman to breastfeed and educating her about healthy weight loss are also important, but these actions come after ensuring the immediate safety and recovery post-surgery.