Final answer:
Kenji's gift of an origami crane to Sadako suggests that he deeply cares for her and wants to encourage hope, according to a traditional story linked with healing. The correct answer is option D.
Step-by-step explanation:
The best inference supported by the passage is D. Kenji cares about Sadako and wants her to feel hopeful. The act of Kenji giving a paper crane and referencing the old story suggests a gesture of care and an attempt to inspire hope. It is not necessarily about the magical powers of the cranes (A), Sadako accepting her fate (B), or Kenji not understanding the severity of the illness (C); instead, it's about Kenji's supportive action to cheer Sadako with a traditional belief that may give her strength and hope during her illness.