Final answer:
The flexible rib cage protects the vital thoracic organs when a child experiences a blunt chest injury.
Step-by-step explanation:
When a child experiences a blunt chest injury, the flexible rib cage protects the vital thoracic organs. Unlike other bones in the body, the ribs have some flexibility, allowing them to absorb and distribute the force of impact without breaking. This flexibility helps to minimize the risk of internal organ damage in the event of a blunt chest injury. Therefore, option D is the correct answer.