Final answer:
The nurse should observe the child's abdominal status, including palpation for splenomegaly and guarding, when monitoring for manifestations of splenic sequestration.
Step-by-step explanation:
When monitoring for manifestations of splenic sequestration in a 6-month-old child experiencing a sickle cell crisis, the nurse should observe the child's abdominal status, including palpation for splenomegaly and guarding (option C). In splenic sequestration, sickled red blood cells become trapped in the spleen, causing it to enlarge rapidly. This can lead to severe anemia and potentially life-threatening complications if not detected and treated promptly.