Final answer:
The best intervention is to limit visitors to immediate family members to reduce infection risk due to the child's low absolute neutrophil count from acute lymphoid leukemia.
Step-by-step explanation:
The nurse creating a plan of care for a child who has acute lymphoid leukemia and an absolute neutrophil count (ANC) of 400/mm3 should include 'A. Limiting visitors to immediate family members only' in the plan. This intervention is crucial because of the child's significantly reduced ANC, which indicates neutropenia and a heightened risk for infection. Limiting visitors can minimize exposure to pathogens and reduce the potential for infections. Other options, such as administering live vaccines and providing a diet rich in fresh fruits and vegetables, would not be appropriate due to the risk of infection and the child's compromised immune system.