Final answer:
Zeus rewards Percy by sparing his life after Percy returns the lightning bolt, thus preventing a war among the gods.
Step-by-step explanation:
In chapter 21 of The Lightning Thief, Zeus rewards Percy by allowing him to live despite Percy's potential threat to his power. This act of clemency comes after Percy successfully returns the lightning bolt to Zeus, thereby preventing a catastrophic war among the gods. Zeus's acknowledgment of Percy's bravery and his decision not to punish him is the reward Percy receives for his heroic actions.