Explanation:
The correct option is 'd'. For children between 6 months and 4-5 years of age (typically under 20 kg) with an isolated, closed diaphyseal femur fracture, early application of a hip spica cast is the safest and most effective standard of care. Flexible intramedullary nails (TENs) are typically indicated for school-aged children (5-11 years), and rigid nails are reserved for older adolescents nearing skeletal maturity to avoid avascular necrosis of the femoral head.

Reference: Flynn JM, Schwend RM. Management of pediatric femoral shaft fractures. J Am Acad Orthop Surg. 2004;12(5):347-359.