Final answer:
A school-age child diagnosed with sickle cell disease in splenic sequestration crisis is most appropriately assigned to a private room.
Step-by-step explanation:
The most appropriate room assignment for a school-age child diagnosed with sickle cell disease in splenic sequestration crisis is a private room (D). This is because the child is in a crisis and requires close monitoring and individualized care. It is important to prevent the spread of infection and contamination, but it is not necessary to isolate the child in a reverse-isolation or contact-isolation room, as these are typically used for patients with contagious conditions.