Final answer:
The appropriate action for the nurse when administering a unit of packed red blood cells to a client with sickle cell disease is to assess vital signs every 15 minutes throughout the entire infusion.
Step-by-step explanation:
The appropriate action for the nurse when administering a unit of packed red blood cells to a client with sickle cell disease is to assess vital signs every 15 minutes throughout the entire infusion (option D). This is important to monitor the client's response to the transfusion and identify any potential adverse reactions. Options A, B, and C are not the correct actions for administering the infusion.